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IC 


8936 



Bureau of Mines Information Circular/1983 




Electron Microscopy Studies 
of Explosion and Fire Residues 



By Daniel L. Ng, Kenneth L. Cashdollar, 
Martin Hertzberg, and Charles P. Lazzara 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8936 



Electron Microscopy Studies 
of Explosion and Fire Residues 



By Daniel L. Ng, Kenneth L. Cashdollar, 
Martin Hertzberg, and Charles P. Lazzara 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 





UNIT OF MEASURE ABBEIEVIATIONS USED 


IN 


THIS REPORT - ^ 
■millibar /jA*V 


A 


ampere 


mbar 




atm 


atmosphere 


mln 




minute i V 
millimeter \j *(AJ 


° C 


degree Celsius 


ram 




° C/sec 


degree Celsius per 
second 


msec 
UA 




millisecond j\» 
microampere 


cm 


centimeter 


pm 




micrometer 


eV 


electron volt 


nm 




nanometer 


g 


gram 


pet, % 




percent 


g/m3 


gram per cubic meter 


sec 




second 


J 


joule 


W 




watt 


K 


kelvin 


W/cm2 




watt per square 


keV 


kilo electron volt 






centimeter 


L 


liter 


W cm-l 


'c- 


watt per centimeter per 
degree Celsius 



This publication has been cataloged as follows: 



Electron microscopy studies of explosion and fire residues. 

(Information circular / Bureau of Mines ; 8936) 

Bibliography: p. 61-63. 

Supt. of Docs, no.: I 28.27:8936. 

1. Mine fires. 2. Mine explosions. 3. Mine dusts— Measurement. 
4. Scanning electron microscope. I. Ng, Daniel L. II, Series: Infor- 
mation circular (United States. Bureau of Mines) ; 8936. 



-TN295.U4 [TN315] 622s [622'. 8] 83-600080 



s^ 



.cs 



^r- 



CONTENTS 

Page 

Abstract 1 

Introduction 1 

Acknowledgments 2 

Experimental apparatus and methods 2 

O*' Optical and electron microscope comparison 2 

Scanning electron microscope 2 

Sample preparation 4 

^ SEM operation 7 

0^ Experimental results 11 

Z^ Dust explosions in an 8-L chamber 11 

^ Pittsburgh coal dust (22-ym) in air 12 

r ^ Pittsburgh coal dust (22-pm) in 50% O2 21 

Pittsburgh coal dust (2-ym) in air. 23 

Pittsburgh coal dust (SA-pm) in air 23 

Particle size effects for Pittsburgh and Pocahontas coals 26 

Pocahontas coal dust in air 29 

Anthracite dust in air and in 50% O2 31 

Graphite dust in 50% O2 34 

Diamond dust in 50% O2 37 

Pittsburgh' coal and rock dust ,in air 40 

Experimental mine dust explosions 43 

Mine explosion disaster investigations 47 

Dust explosions in a 1.2-L furnace 47 

Laser pyrolysis of coal dust 50 

Slow heating of coal dust 54 

Surface characteristics of bituminous and lignite coals 54 

Chemically treated wood 54 

Discussion 57 

Conclusion 61 

References 61 

ILLUSTRATIONS 

1 . Schematic of scanning electron microscope 3 

2. Electron beam interaction region with specimen 3 

3. Scanning electron microscope 5 

4. SEM and associated instrumentation 6 

5. Typical SEM photograph of dust particles 8 

6. X-ray spectra of coal and Purple K dust 9 

^ 7. Potassium X-ray elemental map of dust in figure 5 10 

8. 8-L flammability chamber 11 

fO 9. Coulter Counter size distributions of three classified Pittsburgh coal 

—-^ dusts 12 

*^^ 10. 8-L chamber flammability data for 22-ym Pittsburgh coal dust in air 13 

11. Pressure traces for explosions of 22-ym Pittsburgh coal dust in air 14 

2 — 12. Pittsburgh coal dust (22-jjm), unburned and a low concentration burned in 

air 16 

13. Pittsburgh coal dust (22-ym), intermediate concentrations burned in air... 17 

14. Pittsburgh coal dust (22-ym), high concentration burned in air 18 

15. Pittsburgh coal dust (22-ym), burned in air 19 

16. Optical micrograph of sectioned Pittsburgh coal dust 20 

17. Flammability data from the 8-L chamber for 22-ym Pittsburgh coal dust in 
50% O2 22 



> 



11 



ILLUSTRATIONS—Continued 



Page 



18. Pressure traces for explosions of 22-\im Pittsburgh coal dust in 50% O2.... 23 

19. Pittsburgh coal dust (22-pm), unburned and burned in 50% O2 24 

20. Pittsburgh coal dust (22-)ji]i), burned in 50% O2 25 

21. Flammability data from the 8-L chamber for 2-)jm Pittsburgh coal dust in 

air 26 

22. Unburned Pittsburgh coal dust (2-pm).... 27 

23. Pittsburgh coal dust (2-jjm), burned in air 28 

24. Flammability data from the 8-L chamber for 84-ym Pittsburgh coal dust in 

air 29 

25. Pittsburgh coal dust (84-]jm), unburned and burned in air 30 

26. Lean flammable limit for Pittsburgh and Pocahontas coals as a function of 

particle size 31 

27. Flammability data from the 8-L chamber for 13-)jm Pocahontas coal dust in 

air 32 

28. Pocahontas coal dust (13-jjm), unburned and burned in air............. 33 

29. Flammability data from the 8-L chamber for anthracite in air and in 50% O2 34 

30. Anthracite dust, unburned and burned in air 35 

31. Anthracite dust, burned in 50% O2 36 

32. Flammability data from the 8-L chamber for graphite and diamond dust in 

50% O2 37 

33. Graphite dust (4-ym), unburned and burned in 50% O2 38 

34. Diamond dust (2-|jm), unburned and burned in 50% O2......... •• 39 

35. X-ray spectra of diamond and match particles 40 

36. Flammability data from the 8-L chamber for a Pittsburgh coal and rock dust 

mixture 41 

37. Unburned Pittsburgh coal and rock dust mixture 42 

38. X-ray spectra of limestone rock and coal particles 43 

39. Pittsburgh coal and rock dust mixture, burned in air 44 

40. Unburned Pittsburgh coal and rock dust mixture 45 

41. Pittsburgh coal and rock dust, postexplosion sample from the Bruceton 

Experimental Mine 46 

42. 1.2-L furnace and rapid sampling system..... 47 

43. Pressure and thermocouple recorder traces from an explosion in the 1.2-L 

furnace 48 

44. Dust cloud thermal ignition data from the 1.2-L furnace for 55-)jm Pitts- 

burgh coal in air 49 

45. Unburned Pittsburgh coal dust (55-pm) 50 

46. Pittsburgh coal dust (55-pm), heated in the 1.2-L furnace but before 

thermal ignition 51 

47. Pittsburgh coal dust (55-ym) after an explosion in the 1.2-L furnace...... 52 

48. Laser-pyrolyzed 84-pm Pittsburgh coal at various heating times....... 53 

49. Pittsburgh coal dust slowly heated in a furnace at various temperatures... 55 

50. Comparison of surface characteristics of bituminous and lignite coals 56 

51. Relative distribution of phosphorus as a function of distance from the 

surface, in oak timber chemically treated with NCX inhibitor 57 

52. Relative distribution of phosphorus as a function of distance from the 

surface, in chemically treated oak timber after charring 58 

TABLE 

1. X-ray energies of some common chemical elements 8 



ELECTRON MICROSCOPY STUDIES OF EXPLOSION AND FIRE RESIDUES 

By Daniel L, Ng, ^ Kenneth L, Cashdollar, Martin Hertzberg, 
and Charles P. Lazzara 



ABSTRACT 

This Bureau of Mines report describes the results of scanning electron 
microscopic (SEM) studies of the combustion residues of carbonaceous 
dust particles resulting from explosions and thermal ignitions in labor- 
atory and full-scale mine tests. The dusts studied varied widely in 
rank and volatility: Pittsburgh seam bituminous coal (35% volatility), 
Pocahontas seam bituminous coal (16% volatility), anthracite (7% vola- 
tility), graphite, and diamond. The most systematic explosion studies 
were performed in an 8-L chamber with narrow size distributions of 
Pittsburgh coal dust with surface mean diameters of 2, 22, and 84 ym. 
Experiments were conducted mainly in air, with some data obtained in 50% 
©2 . Observations of the bituminous coal explosion residues showed par- 
ticles with rounded edges, particles with blowholes, and some ceno- 
spheres. The lower volatility dust residues showed fewer changes from 
the original, unburned dust. 

In addition to the experimental studies, results of SEM observations 
of residues from a mine explosion disaster are reported. Comparison of 
such residues with those from the laboratory and experimental mine ex- 
plosions can help investigators determine the extent of dust participa- 
tion in the disaster and in identifying the possible ignition sources. 

INTRODUCTION 

It has been over 70 years since the Bureau of Mines was created by 
Congress and given specific authority to inquire into the causes of gas 
and dust explosions in U.S. mines. In that period, over 1,000 mine ex- 
plosion disasters have been investigated, and an even larger number of 
experimental dust explosion tests have been conducted by the Bureau. 

Since 1979, the scanning electron microscope (SEM) facility has been 
used to study the microscopic structure of dust residues from such ex- 
plosions, to gain fundamental data on the character of these residues 
and provide a basis for determining the extent of dust participation in 
specific mine disasters. Some of the studies of laboratory-scale explo- 
sions reported herein are systematic; others are only examples of the 
capability of the instrumentation and methodology for assisting in field 
investigations. 

^Research physicist. 
^Physicist. 

^Supervisory research chemist. 
Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



ACKNOWLEDGMENTS 



The authors wish to acknowledge the 
contributions of R. S. Conti, W, T. Cor- 
pus, and J, Leff (all of the Pittsburgh 
Research Center) to the experimental work 
in this report. In particular, the au- 
thors wish to thank R. S. Conti for per- 
mission to use his thermal ignition data 



from the 1,2-L furnace (8).^ Special 
acknowledgment is made to P. J. Street of 
the Central Electricity Generating Board 
in the United Kingdom for permitting the 
use of his optical microscopy observa- 
tions ( 35 ) of coal dust residues from the 
Bureau's laboratory-scale explosions. 



EXPERIMENTAL APPARATUS AND METHODS 



OPTICAL AND ELECTRON MICROSCOPE 
COMPARISON 

In all microscopy the important quanti- 
ty is the resolution. In an optical mi- 
croscope (3^, 9^), the diffraction-limited 
resolution for visible light (X = 550 nm) 
is about 350 nm in air, or about 250 nm 
for an oil-immersed lens. The resolving 
power of the unaided human eye is about 
2 min of arc, which corresponds to about 
0.2 mm at a distance of 25 cm. There- 
fore, the maximum practical magnifica- 
tions for the optical microscope are 
about 500 in air and 700 in oil. 



1 to 2 nm. This corresponds to practi- 
cal maximum magnifications of 50,000 to 
100,000 for the SEM. 

Compared to the glass lenses of optical 
microscopes, the electromagnetic lenses 
of the SEM have much greater spherical 
and chromatic aberrations ( 11 , pp. 32-40; 
39, ch. 4). To overcome these imperfec- 
tions, the SEM operates at a large f num- 
ber (or small cone angle) . This also re- 
sults in a large depth of field (about 
100 times the resolution) for the SEM, 

SCANNING ELECTRON MICROSCOPE 



In general, the resolution can be im- 
proved by going to light of shorter wave- 
length. According to quantum physics 
(2) , a particle such as an electron has a 
wavelength that is much shorter than that 
of visible light. The de Broglie wave- 
length X is related to the particle's 
energy by 



^ p (2mE)l/2 



(1) 



where h is Planck's constant, p is the 
particle momentum, m is the particle 
mass, and E is the particle energy. For 
example, a 30-keV electron has a de 
Broglie wavelength of about 0.007 nm, and 
a 200-keV electron corresponds to a wave- 
length of 0.003 nm. Electron microscopes 
( 11 , p. 3; J£, p. 13; 40 ) were made to 
take advantage of the shorter wavelengths 
of electrons to achieve higher resolution 
than optical microscopes. The theoreti- 
cal resolution for a scanning electron 
microscope operating at 30 keV is about 
0.4 nm. In practice the resolution is 
limited by the beam spot size, which is 



Details of the image formation system 
of the SEM appear in the literature ( 11 , 
ch. 4; 40), but a brief description is 
given here. The major components of the 
SEM are illustrated in figure 1, The 
filament is the source of electrons, 
which are accelerated by a high voltage. 
Condenser and objective lenses focus the 
electrons to a fine spot on the sample. 
The scan generator produces a signal 
sweeping in synchronism both the electron 
beam inside the microscope and the elec- 
tron beam inside the cathode ray tube 
(CRT), on which the image of the sample 
under study is displayed. The image is 
produced by modulating the intensity of 
the CRT beam with signals coming from the 
output of the video amplifier. Input 
signals to the video amplifier are usu- 
ally from secondary electrons produced 
by the interaction between the electron 
beam and the sample. The strength of the 
interaction varies from point to point 

"^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



Anode 
Electron beam 



Electromagnetic y^ 
condenser \ens-^ 

Scanning coil 



Electromagnetic 
objective lens- 



Sample 




Filament 
(cathode) 



^ 



mage CRT 




Scan 
generator 



^\I£r^ 



X-ray detector 



Video 
amplifier 



-Electron detector 



FIGURE 1. - Schematic of scanning electron 
microscope. 

according to the local topography and 
chemical composition of the sample. For 
example, the production of secondary 
electrons varies according to the sample 
material and the angle between the beam 
and the sample surface. Image production 
is by a one-to-one correspondence between 
the position scanned by the focused elec- 
tron beam on the sample and the position 
traversed by the electron beam in the 
CRT, The amplitude modulation is the 
variation in secondary electron produc- 
tion rate with respect to position. 
Thus, the secondary electrons are col- 
lected at the detector, where they strike 
a scintillator to produce flashes of 
light. This light signal is guided by a 
light pipe to a photoraultiplier, which 
converts it into electrical signals suit- 
able for input to the video amplifier. 
The magnification is simply the ratio of 
the distances scanned by the two electron 
beams: the one in the microscope, the 
other in the video display. 



Though secondary electrons are most 
often employed to produce images in an 
SEM, other information-carrying signals 
such as backscattered electrons and X-ray 
photons can also be used as input signals 
to the video amplifier to produce images. 
All these signals are produced as a re- 
sult of the interactions between the 
electron beam and the specimen, as shown 
schematically in figure 2, The secondary 
electrons are most often used in high- 
resolution imaging because they are pro- 
duced from an area of the sample only 
slightly larger in diameter than the 
focused spot of the beam, and they 
originate from no deeper than 5 to 50 nm. 
The other signals originate from a much 
larger volume of the beam sample inter- 
action region, resulting in poorer res- 
olution. In spite of their lower resolu- 
tion, images obtained from these other 
signals are useful because they contain 
information of a different kind. For ex- 
ample. X-ray-produced SEM micrographs 
give the spatial distribution of a chemi- 
cal element of interest. This will be 
discussed in more detail in the next 
section. 



* Electron / 
\ beam ' 
\ 



; 



\ / 
\ / 

\dp/ 



-Source of secondary 
electrons Mbsb 




Source of !-c-rays 



FIGURE 2. - Electron beam interaction region 
with specimen. 



The Bureau of Mines Pittsburgh Research 
Center is engaged in mine health and 
safety research. One such area of re- 
search is the prevention of fires and ex- 
plosions in mining-related activities. 
The Center has a JE0L5 JSM U3 scanning 
electron microscope equipped with a solid 
state X-ray detector and multichannel an- 
alyzer to perform energy-dispersive X-ray 
analysis. Figure 3 is a photograph of 
the SEM with the electron gun (filament 
source) at the top, condenser lens and 
scanning coils in the middle, and sample 
manipulation controls at the bottom. 
Figure 4 shows the SEM, console, and as- 
sociated instrumentation. 



sudden streaks of bright lines and dis- 
tortion in the CRT image. The buildup of 
charges also changes the secondary elec- 
tron emission probability and hence the 
signal strength. Methods to overcome 
charging effects usually involve operat- 
ing the SEM at a low accelerating voltage 
or coating the sample with a layer of 
conductive material. Lowering the accel- 
erating voltage usually results in lower 
resolution. The better method is to ap- 
ply a thin layer of conductive material 
on the insulator's surface. The metal 
most often employed is an alloy of gold 
and palladium, and the thickness of the 
coating can be between 10 and 100 nm. 



SAMPLE PREPARATION 

Sample preparation for scanning elec- 
tron microscopy is very simple. If the 
sample is conductive, such as a metal or 
semiconductor, and if it is small enough 
to be introduced into the vacuum chamber 
below the objective lens, no preparation 
is necessary at all. However, care 
should be taken to sufficiently degas the 
sample so that it will not disturb the 
relatively high vacuum (about 10"^ bar) 
requirement of the SEM, It is always 
good practice to have clean samples in 
the SEM, because dirty samples often give 
off hydrocarbon molecules which, under 
intense electron bombardment, produce 
contamination on the surface as a result 
of hydrocarbon cracking and subsequent 
polymerization into a carbon layer. 

Observation of insulating (nonconduc- 
tive) materials can be done without much 
preparation, as in the conductive materi- 
al case; however, the image quality will 
suffer from charging effects. Charging 
is due to the accumulation of beam elec- 
trons absorbed by the insulator because 
there is no conducting path for the ab- 
sorbed electrons to dissipate to the 
electrical ground. This electron accumu- 
lation leads to space charge regions on 
the sample which can actually deflect the 
incident electron beam. This results in 

^Reference to trade names is made for 
identification only and does not imply 
endorsement by the Bureau of Mines* 



In addition to minimizing charging ef- 
fects in SEM images, the application of a 
conductive coating also reduces damage to 
the sample due to heating produced by the 
electron beam. This is particularly im- 
portant in heat-sensitive specimens such 
as biological samples. It can be shown 
(11, p, 513) that the rise in temperature 
AT due to the impact of an electron beam 
of current i (in )jA) with energy E^ (in 
keV) and a beam diameter d (in ym) 
material with thermal conductivity 
W cm-l°C-1) is 



on a 
k (in 



^•8 E„ i 

AT = 2 

k d. 



(2) 



The rise in temperature is only about 
10° C for metals but can be as high as 
1,000° C for plastics. Thus, for exam- 
ple, an aluminum coating 20 nm thick can 
reduce the value of AT from 1,430° C to 
515° C for some nonconductive samples 
(^, p, 513), 

Coal can sometimes be classified as an 
electrical semiconductor, but in scanning 
electron microscopy, it behaves more like 
an insulator, Uncoated coal samples ex- 
hibit strong charging effects. To obtain 
quality images, they need to be conduc- 
tively coated. 

If the material to be studied contains 
a large quantity of water, it is neces- 
sary to have the water removed. This can 
be done simply by drying. If the fea- 
tures of interest are delicate, special 




FIGURE 3. - Scanning electron microscope. 




FIGURE 4. - SEM and associated instrumentation. 



methods of drying have to be used, be- 
cause ordinary drying occurs by transport 
of water across a liquid-vapor interface. 
Surface tension of the receding liquid 
can exert a very large force per unit 
area, and the result is that delicate 
microscopic features can be squashed and 
collapsed. To prevent this, special 
methods such as freeze drying and criti- 
cal point drying should be used (11, 
pp. 495-505). Since these considerations 
are unimportant with respect to coal dust 
and other dusts whose investigations are 
reported here, we will not go into detail 
concerning them. It is generally suffi- 
cient to air-dry coal dust samples. 



been collected, they are prepared essen- 
tially as follows. Dust samples are 
transferred using a microspatula onto a 
vinyl tape coated with adhesive on both 
sides. This tape is chosen over a simi- 
lar cellophane tape because of its low 
outgas characteristics, which make it 
compatible with the SEM's high-vacuum re- 
quirement. The tape is mounted on an 18- 
mm-diameter copper or aluminum disc. If 
necessary, the quantity of dust sampled 
can be determined by weighing with an 
electronic balance. For solid samples, 
the sample is cut to a suitable size 
(usually less than 25 mm in maximum 
dimension) and mounted similarly. 



Samples are collected in several ways 
as will be described. Once they have 



The application of a layer of conduc- 
tive material is usually accomplished by 



sputtering or evaporative deposition. 
Sputtering is done by placing the pre- 
pared sample inside the chamber of a 
sputter coater. The chamber is evacuated 
to a low pressure. By using a leak 
valve, a steady vacuum of about 0.13 mbar 
is maintained. A plasma discharge is 
produced by the application of a high 
voltage between the sample and a metallic 
anode. Positive ions that are produced 
by the gas discharge impact on the anode 
and sputter a metallic coating from the 
anode target (usually a gold and platinum 
alloy) onto the sample. The thickness of 
the metallic layer can be controlled by 
varying the magnitude of the discharge 
current, the voltage setting, and the 
length of sputtering time according to 
calibration curves. Coating by evapora- 
tive deposition is done inside a vacuum 
evaporator. A measured quantity of Au-Pd 
alloy wire is placed inside a tungsten 
wire basket, which is supported on elec- 
trodes. The sample is placed on a plane- 
tary rotating platform below the wire 
basket, and the whole assembly is en- 
closed inside a bell jar evacuated to a 
vacuum of about 10"^ mbar. A high cur- 
rent (20 to 30 A) is passed through the 
tungsten basket to melt and then evap- 
orate the Au-Pd wire. A fine mist of 
vaporized metal is produced which con- 
denses onto the sample to produce the 
desired coating. Coating by the evap- 
oration of a metal is the preferred 
method when working with heat-sensitive 
materials. 

SEM OPERATION 

When a sample is studied in the SEM, it 
is observed at a magnification appropri- 
ate for the information to be sought, 
usually from 100 X to 10,000 X. For mag- 
nifications above 500 X, it is the higher 
resolving power of the SEM that makes it 
more useful than the optical microscope. 
Below 500 X, its larger depth of focus 
allows one to observe features not possi- 
ble with the light microscope. There are 
usually two imaging modes for the SEM 
imaging, the slow scan mode and the TV 
scan mode. In the TV mode, the TV scan 
generator sweeps the electron beam over 



the observed surface about 30 times a 
second; i.e., 30 TV picture frames are 
presented to the eye in 1 sec. This 
rapid scan speed allows the eye to follow 
motions from one region of the sample to 
another because the image follows the 
sample movement instantaneously. This is 
not the case in the slow scan mode, where 
it takes up to tens of seconds to scan a 
particular region. However, the ability 
to rapidly scan a region of the sample at 
the TV rate is achieved with a loss of 
resolution and image quality. This is so 
because, in the fast TV scan mode, the 
electron beam does not dwell on a spot 
long enough to collect enough electrons 
to obtain a good signal-to-noise ratio. 
Therefore, in the TV scanning mode, the 
signal-to-noise ratio must be improved by 
increasing the beam spot size. This in- 
creases the number of electrons striking 
the sample but decreases the resolution. 
To increase the signal-to-noise ratio and 
at the same time maintain high resolution 
requires a longer sampling time. This is 
achieved in the slow scan mode, which is 
the mode one employs to photograph the 
image. A recording time of 50 to 100 sec 
is typical. Consequently, the normal 
practice is to use the TV mode to locate 
the features of interest and then to 
switch over to the slow scan mode and 
adjust the various electronic controls 
to obtain the best contrast, brightness, 
and image sharpness. The image is then 
recorded photographically on Polaroid 
type 52 positive or type 55 positive- 
negative film. Sometimes it is advan- 
tageous to record stereo pairs, which 
are two micrographs of the same area, 
viewed 5° to 7° apart by rotating the 
tilt of the specimen stage. When this 
pair of micrographs is viewed with the 
aid of stereo viewer, a lifelike three- 
dimensional-appearing image of the origi- 
nal object results. 

As discussed previously, the images 
produced by secondary electrons have the 
highest resolution. All of the SEM pho- 
tomicrographs in this report were made 
using an accelerating voltage of 25 keV 
for the incident electron beam and sec- 
ondary electrons for imaging. 



Figure 5 is a typical SEM photomicro- 
graph of a sample containing coal dust 
and a dry chemical fire extinguishing 
agent called Purple K (fluidized KHCO3), 
Often times it is desirable to know the 
chemical composition of a sample, or 
where in the sample a given element or 
impurity is located, A nondestructive 
method of determining the chemical or el- 
emental composition of a sample is the 
electron probe microanalyzer (EPMA) , 
This is an instrument in which the sample 
is bombarded with electrons of suffi- 
ciently high energy (i.e. , with kinetic 
energy higher than the bonding energy of 
the inner orbital electrons of the atoms 
making up the sample) so that some of 
these orbiting electrons will be eject- 
ed from the atom, leaving it in an ex- 
cited state. The excited atom relaxes 
by having an electron from an outer 
orbit fall into the vacancy left behind 
by the ejected electron. According to 
quantum theory, the difference in en- 
ergy between the initial and final state 
of the electron making this transition 
causes the emission of a characteris- 
tic X-ray, called the K, L, or M line 




Pittsburgh coal 

and 

Purple K 



100 



Scale, yuvn 



depending on whether the vacancy was cre- 
ated in the K, L, or M shell (11, pp. 69- 
78; 22.' PP* 243-247). (K, L~ M, etc., 
are names given to the electrons in dif- 
ferent orbits around the atom.) Using 
suitable instrumentation, these X-rays 
are collected and analyzed by the EPMA, 
It was discovered by Moseley ( 11 , p. 74; 
28 ) in 1913 that the characteristic X- 
rays are related to the atomic number Z 
of the bombarded atoms by the following 
equations: 

A = J^ (3) 

or 

E cc (Z-a)2 (4) 



where X and E are the wavelength and en- 
ergy of the X-ray photons and K and a are 
semiempirical constants. Thus the pres- 
ence of X-rays of a particular energy in- 
dicates the presence of an atom of a par- 
ticular atomic number, i.e. , a particular 
element. In an electron probe micro- 
analyzer, the concentration of elements 
in a sample can be quantitatively deter- 
mined from the intensity of the lines. 
The X-ray energies of the common elements 
of interest are found in table 1, The 
K^-line results from an electron falling 
from the L-shell to the K-shell, and the 
Ko-line results from an electron falling 
from the M-shell to the K-shell, 

TABLE 1, - X-ray energies of some common 
chemical elements 



FIGURE 5. - Typical SEM photograph of dust 
particles. 



Atomic 


Name 


Sjmibol 


Energy, keV 


number 


Ka 


J^P 


(Z) 










11 


Sodium 


Na 


1,04 


1.07 


12 


Magnesium, , , 


Mg 


1,25 


1.30 


13 


Aluminum. . . . 


Al 


1,49 


1.55 


14 


Silicon 


Si 


1,74 


1.83 


15 


Phosphorus. . 


P 


2.01 


2.14 


16 


Sulfur 


S 


2.31 


2,46 


17 


Chlorine, , , . 


CI 


2,62 


2,82 


19 


Potassium. . . 


K 


3.31 


3.59 


20 


Calcium 


Ca 


3.69 


4.01 


22 


Titanium,,,, 


Ti 


4.51 


4.93 


24 


Chromium, , , , 


Cr 


5.41 


5.95 


26 


Iron 


Fe 


6.40 


7.06 


27 


Cobalt 


Co 


6.92 


7.65 


28 


Nickel 


Ni 


7.47 


8.16 


29 


Copper 


Cu 


8.04 


8.90 



As mentioned earlier, the SEM has an 
electron beam-specimen interaction re- 
gion which produces X-rays in addition 
to secondary electrons. The mechanism 
for the X-ray production is exactly the 
same as in the electron probe micro- 
analyzer. If these X-rays are collected 
and analyzed according to wavelength or, 
equivalently , according to energy, the 
SEM is actually an EPMA. Almost all 
SEM's are equipped with X-ray analyzers. 
There are two kinds of X-ray analyzer in 
use: wavelength-dispersive and energy- 
dispersive analyzers. The wavelength- 
dispersive analyzer ( 11 , pp. 263-274; 39 , 
pp. 252-256) uses crystals to diffract 
the X-rays according to Bragg 's law. The 
diffracted X-rays are detected using a 
proportional counter, and the elements 



are identified according to Moseley's law 
(equation 3) . For the energy-dispersive 
analyzer ( 11 , pp. 274-281; 29, PP. 265- 
272) , X-rays pass through a thin beryl- 
lium window into an evacuated chamber 
where they are detected by a liquid- 
nitrogen-cooled, lithium-drifted silicon 
crystal. When X-ray photons are absorbed 
in the detector's crystal, electron-hole 
pairs are created and are collected by an 
applied electric field to produce pulses 
which are amplified and then processed by 
a multichannel analyzer where they are 
sorted according to voltages. The pulse 
voltages are proportional to the energy 
of the incident X-rays. The energy- 
dispersive analyzer cannot detect X-rays 
of elements whose atomic number Z is less 
than 11 because the detector window. 




4 5 6 7 

ENERGY, keV 

FIGURE 6. - X-ray spectra of coal and Purple K dust. 



8 



10 



10 



usually made of beryllium about 10 ym 
thick, strongly absorbs these lower en- 
ergy X-rays. Also the characteristic X- 
rays of the elements of 4 < Z < 7 are 
only about 100 to 150 eV apart, and the 
energy resolution of solid state detec- 
tors is usually not sufficient to resolve 
these X-rays, The Bureau detector has a 
resolution of about 180 eV at the 6.40- 
keV iron line. To analyze X-rays below 
atomic number Z = 11 (i.e., for carbon, 
oxygen, nitrogen, etc.), the wavelength- 
dispersive analyzer is recommended. 

If all the X-rays emitted from the sam- 
ple are collected and analyzed by energy, 
spectra like those shown in figure 6 are 
obtained. These particular X-ray spectra 
are from two of the dust particles shown 
in figure 5. Qualitative analysis of the 
elemental composition of the sample is 
trivially done by noting whether X-rays 
of energies corresponding to a particular 
element are present in the spectrum. The 
strong line at 3.3 keV in figure bA is 
the potassium K^^-line; this must be a 
Purple K particle since Purple K is com- 
posed mainly of potassium bicarbonate, 
KHCO3. The other particle has no strong 
lines (except for the K-line of copper at 
8.0 keV due to the copper sample holder 
and the M-lines of gold and platinum at 
about 2.1 keV from the coating). The 
other particle must therefore be a coal 
particle, since its main component, car- 
bon, has too low an energy to be ob- 
served. With suitable instrumentation, 
such as a small dedicated computer, quan- 
titative information concerning the con- 
centration of elements could also be 
obtained. 

The X-ray spectrum can be accumulated 
with the electron beam scanning over the 
entire region of the sample being exam- 
ined by the SEM, or with the electron 
beam directed towards a single spot on 
the sample. The spectrum accumulated 
in the scanning mode would give an indi- 
cation of the elements that are present 
in the group of particles in the region 
under observation, whereas that of the 
spot mode would give a spectrum of only 
that particle or portion of the particle 
that is present in the localized spot. 



Comparison of spectra from different par- 
ticles would give semiquantitative infor- 
mation as to the relative concentra- 
tion of elements in each by comparing 
the relative intensities of their X-ray 
lines. 

A map of the distribution of a partic- 
ular element throughout the sample can 
be obtained by collecting only the X-rays 
that originate from the element of in- 
terest and using these signals instead 
of the secondary electrons to produce 
an image. For example, figure 7 is an 
X-ray map using only the potassium K- 
line, 3.3 keV, shown in figure 6a» A dot 
in figure 7 represents the presence of 
potassium atoms. By comparing figures 5 
and 7, the various particles shown in 
figure 5 can be identified as either Pur- 
ple K or coal, since only the Purple K 
particles appear in figure 7. Elemental 
X-ray maps provide information about the 
spatial distribution of the element of 
interest in relation to the rest of the 
sample. Elemental distribution measure- 
ments are not limited to potassium but 
can be made for any other element by 
selecting its characteristic line. 




Potassium 
X-ray map 



100 



Scale, ji m 

FIGURE 7. - Potassium X-ray elemental map of 
dust in figure 5. 



EXPERIMENTAL RESULTS 



11 



DUSTS EXPLOSIONS IN AN 8-L CHAMBER 

These studies are part of the research 
activities of the Bureau of Mines and 
were performed in conjunction with stud- 
ies of the f lammability of coals and 
other dusts. Laboratory-scale studies of 
dust f lammability in an 8-L chamber have 
been reported previously (13, 16-17). A 
schematic drawing of the chamber and air 
dispersion tank is shown in figure 8. 
The drawing shows the position of an op- 
tical dust probe (_7, 23) , pressure trans- 
ducer, oxygen sensor, dust cup, and igni- 
tion point. The top plate of the chamber 
is usually fitted with a sapphire window 
assembly (not shown), through which the 
infrared radiance of the explosion can be 
measured (4^-_5) . The normal procedure is 
to spread a measured mass of dust uni- 
formly around the disperser cone. The 
top plate is then bolted on, and the 
chamber is partially evacuated to about 
0.2 atm. The air-dispersion tank is 



-30.5- 



'////>//////////// ////\y/r n -Gasket 



33.5 



-19.5- 




. 1 I u^ Pressure 

1^ I transducer 



-Optical dust probe 



Electric match ignition 
source 

Dust cup and disperser 
cone 



All dinnensions in 
centimeters 



-Check valve 



Gasket 



-Solenoid valve 



20 



z:Jfezzzz:i 



Air dispersion tank 



^— 2^^To air 
•~^ supply 



A/ /////////////// /-T-yy 



-30- 



pressurized (to about 6 atm); when the 
solenoid valve between it and the pre- 
evacuated chamber is opened, an intense 
air-jet impulse is injected. This air 
impulse disperses the dust, mixes with 
it, and raises the chamber pressure to 
1.0 atm absolute. The dispersion air 
pulse lasts for 0.2 sec, and after 
another 0.1-sec delay to allow for more 
uniform dispersion, the ignition source 
is energized. The standard ignitor is an 
array of four electrically activated, 
pyrotechnic matches, which deposit their 
energy (140 J) in about 0.05 sec. If the 
mixture is flammable, the developing 
pressure and infrared spectral radiance 
,are monitored. When flame propagation is 
complete and after the combustion prod- 
ucts cool, the residual oxygen content is 
measured and dust or gas samples may be 
collected for analysis. 

Dust flammability has been studied as 
a function of dust volatility, particle 
size, initial oxygen concentration, and 
added inhibitor dusts. Size distribu- 
tions for three classified Pittsburgh 
seam coal dusts used in these studies 
are shown in figure 9. The data are 
from Coulter Counter size analyses and 
are plotted as a surface-area-weighted 
distribution using a semi logarithmic 
plot. The smallest dust has a surface 
mean diameter D„ = 2.2 pm, a mass mean 



D, 



.^ = 2.6 ym, 
standard deviation 



diameter 

ric 

intermediate-size dust 

D^^ = 32 ym, and Og = 1.82. 



and 



Og = 1.46. 
has Dg 



dust has Dg = 84 



FIGURE 8. - 8-L flammability chamber. 



geomet- 
The 
= 22 ym. 
The coarsest 
ym, D^^ = 89 ym, and Og 
= 1.14. The two finer size distributions 
were obtained with a centrifugal clas- 
sifier, and the coarsest dust was sieve- 
classified (200 X 140 raesh). The data 
in figure 9 are for unagglome rated dusts 
since they were dispersed in alcohol 
for the Coulter Counter size analyses. 
When dispersed in air, the dusts tend 
to agglomerate to somewhat larger sizes 
(7, 16), but this agglomeration does not 
affect the flammability behavior signifi- 
cantly. These narrow size distributions 
of coal dust make it much easier to ob- 
serve with the SEM any size changes that 
might occur during an explosion. 



12 



60 



Z 50 

CL 



g 

I- 

CD 

(T 

I- 
CO 

Q 

Q 30 

UJ 



I 
liJ 

UJ 
U- 

q: 

3 



20 - 



10 



I'll 



03=2.2 /im- 




0.5 



1 



J L 




D- = 22^m 



100 200 



12 5 10 20 

DIAMETER, /im 
FIGURE 9. - Coulter Counter size distributions of three classified Pittsburgh coal dusts. 



Pittsburgh Coal Dust (22-}jm) in Air 

Figure 10 shows f lammability data for 
explosions of the 22-ym Pittsburgh seam 
coal dust in air in the 8-L chamber. 
This coal had an "as received" volatility 
of about 35% according to the ASTM D3175 
standard test (1), although the real vol- 
atility under the faster heating rates of 
explosions would probably be higher (19). 
The data include temperatures, oxygen 
consumed, and pressures as a function of 
dust concentration. The measured pres- 
sure ratio in figure lOc was the maximum 
explosion pressure (absolute) divided by 
the initial pressure (about 1 atm abso- 
lute) , with a small correction for the 
pressure rise caused by the matches them- 
selves. The data show no flame propaga- 
tion, and therefore no pressure rise. 



below 100 g/m^. Between 125 and 175 g/m^ 
there is a rapid increase in the explo- 
sion pressure. In previous publications 
(13, 16), the authors chose a pressure 
ratio of 2.0 as a criterion of signifi- 
cant flame propagation. That criterion 
gives a lean flammable limit of 130 g/m^ 
for Pittsburgh coal dust. At the higher 
concentrations, the pressure curve levels 
off as all the oxygen in the air is con- 
sumed. This is shown in figure IO5, 
which is a graph of the decimal fraction 
of the oxygen in the air that is consumed 
during the explosion. The decimal frac- 
tion was calculated from the measured 
amount of oxygen left in the chamber af- 
ter the explosion. 

The pyrometer temperatures shown in 
figure 10a were calculated at the time of 



13 



UJ 

on 
cr 

UJ 
Q. 

lU 



2,800 

2,600 

2,400 

2,200 

2,000 - 

1,800 

1,600 





1,400 




1.0 


u 




UJ 




s 


.8 


3 




V) 




z 


.6 


o 




c> 




z 


.4 


UJ 




^ 


.2 


X 




o 







7 



- 


1 
/J 




1 




1 


1 




1 1 1 1 
KEY 


- 


- 










a 






D— -D Gas ] ^ 

• — • Dust] 6X pyrometer 


- 


- 








*«*^N 


'v 






X K Dust, 3X pyrometer 


- 










D 


\ 
\ 

\ 




















> 


D 

\ 








— 










D 


\ 

\ 






— 


~ 








• 
X 




\ 


a 




■ 


- 




X 




y^X 


"N{ 


X ° 


s 

s 




- 






• 


/ 


• 


x\ 

• 


^s« 


% 


% D 

V 










A 




• 


\* 


• 


V 
^ 




- 




x/ 

X / 


• 






X 




• ^^^ 


- 


- 




/ 












- 






xl 












* X 




- 




• /• 












• 


- 


- 


1 


i 


i 




1 


1 




1 1 1 1 


- 







~T 






1 


y^ 


1 


-^"^ 


* 


-r* 1 


1 




B 








•/ 


/"• 




















/ 


/• 














- 




^ 


; 


i 

• • 


1 




1 






1 1 1 


1 



g 

I- 
< 
a: 

UJ 

q: 

tn 
w 

UJ 

vc 

Q. 




COAL DUST CONCENTRATION, g/m^ 

FIGURE 10. - 8-L chamber flammability data for Tl-]im Pittsburgh coal dust 
in air. 



14 



maximum radiance for each explosion. The 
particle temperatures were calculated 
from the infrared continuum radiance mea- 
sured by a three-wavelength (3A) pyrom- 
eter (4^-5) and by a six-wavelength (6A) 
pyrometer (5-6). The 6X pyrometer also 
measured the gas temperature in the ex- 
plosion by observing the 4.4-jjm CO2 emis- 
sion band radiance. For the data of 
figure 10, the maximum dust particle tem- 
perature and the maximum gas temperature 
occur at a concentration of about 250 
g/m^; this value is close to the concen- 
tration at which the burning velocity is 
a maximum for dusts in this size range 
( 18 , 33-34) . It is also approximately 
the concentration at which the amount of 
combustible volatiles from the coal is at 
a stoichiometric ratio with respect to 
the air. The temperatures measured in 
this constant-volume chamber are higher 
than those of atmospheric flames due to 
adiabatic compression. The gas tem- 
peratures for these explosions are from 
one hundred to several hundred degrees 
higher than the dust particle tempera- 
tures. This tends to confirm a combus- 
tion mechanism in which the coal parti- 
cles devolatilize, generating hydrocarbon 
gases that burn in the gas phase. The 
dust particles remain cooler because of 
their continuing endothermic pyrolysis 
and the fact that the seat of the exo- 
thermic reaction is in the gas phase at a 
significant distance from the particle. 

Dust samples for SEM analysis were col- 
lected for numerous explosions of the 22- 
ym Pittsburgh coal dust in the 8-L cham- 
ber. Pressure traces as a function of 
time for several of these explosions are 
shown in figure 11. As the concentration 
increases, both the maximum pressure and 
the rate of pressure rise increase. 

To answer the question of what actually 
happens to each coal dust particle during 
the course of an explosion, one must com- 
pare the features and appearances of 
dusts before and after the explosion. 
After an explosion has been set off in 
the 8-L vessel, ample time is allowed for 
the dusts to settle. A small quantity of 
dust is then collected with a spatula. 



usually from the surface layer at the 
bottom of the vessel. Sometimes at low 
dust concentrations, almost all the dust 
appears to be consumed. In that case, 
vinyl tape coated with adhesive on both 
sides is applied to the wall of the ves- 
sel to pick up the dust residue that ad- 
heres to the wall. These samples are 
then prepared for SEM examination. 



4 
2 



"> \ r- 



/i Cm=l25g/m- 



J L 



J I L 




0.2 0.4 0.6 0.8 
TIME AFTER IGNITION, sec 

FIGURE 11. - Pressure traces for explosions 
of 22-//m Pittsburgh coal dust in air. 



15 



In the series of experiments using the 
classified Pittsburgh coal (D^ = 22 ym) , 
the dust concentrations were varied from 
125 to 700 g/xsr" ' The variations of tem- 
perature and peak pressure ratio as a 
function of concentration are shown in 
figure 10, and the pressure-time traces 
of these experiments appear in figure 11. 
The 125-g/m^ concentration was a partial 
explosion, with a recorded pressure ratio 
of 1.9. Because of the small amount of 
dust used, very little of it was left 
behind to be collected, and the double- 
adhesive-tape procedure was used to sam- 
ple the residues. SEM micrographs ob- 
tained at different magnifications from 
dusts before and after the explosion are 
shown in figure 12, Figures \2A-12C are 
the unburned or bef ore-explosion dusts at 
three magnifications; figures llD-llF are 
the residue particles from the test at 
125-g/m3 concentration. (The mass con- 
centration of dust is denoted by C^ in 
the figure.) Notice the following fea- 
tures of figure 12: 

1. After the explosion, the particles 
are noticeably larger than before the 
explosion. They exhibit rounded and 
smoothed surfaces in contrast to the 
angular and sharp edges of the particles 
before the explosion, 

2. The particles having undergone ex- 
plosions also have blowholes, and some 
have formed cenospheres (29). 

3. Some of the particles after explo- 
sion, e.g. , those in the upper middle 
section of figure 12/?, are practically 
the same as those before the explosion. 
Thus a small number of particles are al- 
most untouched by the explosion. It has 
been known that in explosions with low 
pressure ratios, the explosion fireball 
propagates preferentially upward and 
along certain directions in which the lo- 
cal dust concentration is higher than the 
average dust concentration. Particles in 
other directions will thus be untouched. 

Those particles that are involved 
in the explosion are subject to rapid 



heating. Absorption of energy by the 
particles causes softening, resulting in 
a plastic state. Surface tension begins 
to smooth the edges and to produce the 
minimum surface energy configuration, 
which is a sphere. If two such particles 
should come into contact, these surface 
tensional forces cause them to adhere. 
Continued rapid heating subsequently 
leads to outgassing of volatiles, which 
are then burned, contributing to the ex- 
plosion through rapid release of energy. 
This outgassing causes the deformed coal 
particle to swell and balloon and even- 
tually leads to the formation of blow- 
holes of various sizes, A variety of 
devolatilization models for bituminous 
coals have been proposed (38), 

For higher dust concentrations, the re- 
sulting explosions produce progressively 
higher pressure ratios. The flame tem- 
peratures first increase rapidly as the 
concentration increases above the lean 
limit but then level off at the higher 
concentrations. Data were also obtained 
for the same 22-|jm Pittsburgh coal dust 
from explosion tests at the higher con- 
centrations of 175, 250, 400, and 700 
g/m^. Typical SEM micrographs of dust 
residues from these explosions are dis- 
played in figures 13, 14, and 15. As the 
dust concentration increases, the frac- 
tion of dust involved in the explosion 
increases. As shown in figures 13^-13C, 
none of the dust residue particles from 
the test at 175 g/m^ has any resemblance 
to the original dust. The same is true 
for the 250- , 400- , and 700-g/m^ concen- 
trations. The degree of agglomeration 
increases with concentration. At the 
highest dust concentration explosion, 
huge lumpy materials were collected, as 
shown in figure 15z?, which is an optical 
microscope image at low magnification. 
These agglomerated masses bore no resem- 
blance to the original dusts. At these 
higher concentrations, near the peak of 
the explosion pressure when all the par- 
ticles are rapidly heated and becoming 
plastic, the probability for collision 
between particles is high, and the colli- 
sions result in large agglomerates. 



16 





A Unburned 



300 
J 



Scale, /im 



D Burned 
Cm » 125 g/m^ 



300 



Scale, ^m 





B Unburned 



100 



Scoic, /ifn 



E Burned 

Cm=l25g/m3 



IQO 



Scale, ^m 





C Unburned 



30 



Scale, y.m 



F Burned 



Scale, f(.m 



FIGURE 12. - Pittsburgh coal dust (22-^m), unburned and a low con= 
centration burned in air. 



17 









4 

r 









A Burned 
Cm=i75 g/m^ 



300 



Scole, ^m 



D Burned 



300 



Scole, ftm 





B Burned 

Cm = 175 g/m? 



Scale, fifn 



100 



E Burned 

Cm=250 g/m-^ 



100 



Scale, /ifA 





C Burned 





i 1 

Scole, /J m 



100 



F Burned 

Cm=250 g/m^ 



Scoie, ^m 



100 
J 



FIGURE 13. - Pittsburgh cool dust (22-/im), intermediate concentra- 
tions burned in air. 



18 




A Burned 

C„,=400 g/m^ 



3Q0 



J i.„ 




Scole, jji.m 



C Burned 







iOO 



Scole, /xm 




B Burned 

Cm=400 q/rx? 







300 




D Burned 

C^=400g/m3 







100 



Scale, /^m u^=tuug/m- Scale, /u.m 

FIGURE 14. - Pittsburgh coal dust (22-fim), high concentration burned in air. 



19 





A Burned 



300 



Scole, fim 



D Burned 

Cm=700g/m' 



t,500 



Scole, ^m 





B Burned 
C,n=700g/m3 



100 



Scale, ^LX<\ 



E Burned 

Cm=700 g/m^ 



Scale, ^m 



100 
J 





C Burned 
Cm-70Dg/m3 



30 



Scale, ym 



F Burned 

Cm=700 g/m^ 



30 



Scate, ftm 



FIGURE 15. - Pittsburgh cool dust (22-^m), burned in air. [D , opti- 
cal microscope photograph.) 



20 




A Burned in air 



6 Burned in air 
C^=400 g/m^ 




I- 



20 

-J 



Scale, fxm 






L 



50 



Scale, ^m 



FIGURE 16. - Optical micrograph of sectioned Pittsburgh coal 
dust (35). 



21 



Occasionally, at the higher magnifica- 
tion micrographs (see figure 13^, lower 
right corner) , one can notice some parti- 
cles that are almost perfect spheres with 
sizes varying from 1 to 20 pm in diam- 
eter. These particles were not derived 
from the coal dusts but came from the 
electric match ignition source. When the 
matches are set off, their component 
metal compounds such as lead and cerium 
are vaporized and then recondensed. The 
almost perfect spherical shapes are pro- 
duced by the condensing metals. These 
spherical droplets were identified as 
metals by their characteristic X-rays in 
a separate experiment in which only elec- 
tric matches were set off in the 8-L 
chamber. The particles collected after 
that experiment showed the same shape as 
the type of particle shown in the lower 
right corner of figure 13f and gave sim- 
ilar X-ray spectra. 

Dust residue samples from the explo- 
sions shown in figures 12-15 were also 
sent to P. J. Street of the Central Elec- 
tricity Generating Board Marchwood Engi- 
neering Laboratories in England. His op- 
tical microscopy observations (35) of 
sectioned explosion residues are briefly 
reported here with his kind permission. 

The dust samples were vacuum-dried at 
105° C. Then, a few milligrams were 
mounted in epoxy resin, cross-sectioned, 
and polished before being examined under 
an optical microscope using reflected 
light. At the lower dust concentrations, 
many of the residue particles were un- 
changed; others were thick -walled hollow 
spheres or C-shaped partial spheres. 

At the higher concentrations, almost 
all of the residue particles were changed 
from their original shapes. Figure 16 
shows sectioned particles from the resi- 
due of the 400-g/m^ coal dust explosion 



and can be compared to the SEM micro- 
graphs in figure 14. One particle in 
figure 16a is a thick walled cenosphere; 
the other particle shows internal bub- 
bles , probably formed by the rapid gen- 
eration of volatiles that had not yet 
vented. The particle in figure 165 is 
a thin-walled "balloon" or cenosphere. 
Street also observed "agglomerated bal- 
loons" up to 500 pm in size with "net- 
works of partitions," which probably 
correspond to the walls of the origi- 
nal particles before they agglomerate 
as in figure 145; the sectioning by 
Street shows the internal structure, 
while figure 145 shows only the external 
structure. 

Pittsburgh Coal Dust (22-ym) 
in 50% O2 

A similar series of 8-L explosion ex- 
periments using the same 22-)jm Pittsburgh 
coal dust was performed in an atmosphere 
consisting of 50% O2 and 50% N. The var- 
iations of particle temperature (from 6A 
pyrometer data) and peak pressure ratio 
as a function of dust concentration are 
shown in figure 17. The measured lean 
limit in 50% O2 was about 90 g/m^. The 
explosion pressure ratios and particle 
temperatures are higher at all concentra- 
tions than for explosions of the same 
dust in air. The pressure-time traces of 
three of these explosions are shown in 
figure 18. Except for the test with 350- 
g/m3 dust concentration, not enough dust 
residue remained in the bottom of the 
chamber to be collected by a spatula. 
However, using the double-adhesive-tape 
sampling method, dust samples from the 
lower concentrations were obtained for 
SEM observation. Figure 19 shows the 
dust residue particles from a 100-g/m^ 
dust explosion compared to the original 
dust particles at three magnifications. 
The dust particles show signs of having 



22 



u 2,600 




100 150 200 250 300 350 400 
COAL DUST CONCENTRATION, g/m^ 

FIGURE 17. - Flammability data from the 8-L chamber for 22-^m Pittsburgh coal dust in 
50% 2. 



gone through an explosion since they have 
blowholes and smoothed edges or corners, 
but the original shape of the coal parti- 
cles is sometimes still discernible. 
Figure 20 shows the SEM micrographs of 
250- and 350-g/m^ dust explosions. It is 
obvious that all the particles shown have 
gone through explosions, with extensive 



blowholes , cenospheres , and heavy agglom- 
eration. These explosions in 50% O2 pro- 
duced higher explosion pressure ratios, 
higher temperatures , and more rapid re- 
actions. The formation of a plastic 
state followed by devolatilization and 
outgassing must occur at a faster rate 
than in air. The highest concentration 



23 




1 1 r 



B Cm = 250g/m^ 




u 
8 


■ T-" r ' 


1 ' 1 1 1 1 

C Cm = 350g/m^^ 


6 


- X^ 


- 


4 


- 


^^^-^__^^- 


2 


i 

1 


1,1,1, 



0.2 0.4 0.6 0.8 

TIME AFTER IGNITION, sec 



1.0 



FIGURE 18. - Pressure traces for explosions of 
22-^m Pittsburgh coal dust in 50% Oj. 

studied, 350 g/m^ , is still a lean mix- 
ture in 50% O2 ; hence the pressure ratio 
was still increasing (fig. 175) at that 
concentration, 

Pittsburgh Coal Dust (2-ym) in Air 

Figure 21 shows the variation of explo- 
sion pressure with dust concentration for 
the 2-ijm Pittsburgh coal dust (D3 = 2.2 
ym in figure 9). Note that these data 
are very similar to those in figure lOC 



for the 22-ym dust. This is a very 
small-sized dust, as shown in the Coulter 
size distribution in figure 9 and the SEM 
micrographs of the unburned dust in fig- 
ure 22. This dust is clearly much small- 
er than that used in the previously de- 
scribed experiments. SEM micrographs of 
this finer dust after a 200-g/m3 explo- 
sion are shown in figure 23. A notice- 
able feature is that all the dust resi- 
dues are considerably larger than the un- 
burned dust. Blowholes and cenospheres 
abound. In many cases, the original par- 
ticles have agglomerated to huge masses 
with diameters of the order of 100 ym. 
Since these large spheres show no trace 
of the original particles, they must have 
been formed while the particles were 
still plastic. 

Pittsburgh Coal Dust (84-ym) in Air 

A few tests were performed with Pitts- 
burgh coal dust of much larger size, Dg 
= 84 ym in figure 9. The pressures and 
temperatures are plotted as a function of 
concentration in figure 24. Note that 
the lean flammable limit for this dust in 
air is 210 g/m^, considerably higher than 
for the two smaller sizes of Pittsburgh 
coal. Also, the maximum explosion pres- 
sures and maximum particle temperatures 
are lower than those of the smaller 
dusts. SEM micrographs of the unburned 
and burned dust particles are shown in 
figure 25. The characteristic angular 
features of the unburned dust are clear- 
ly visible (figs. 25^-25c). Micrographs 
of the dust residues after a 300-g/m3 
explosion (figs. 25Z?-25f) showed that 
practically every particle has reacted 
at this concentration. The angular fea- 
tures are totally absent after explosion, 
indicating that the particles have under- 
gone considerable heating and devola- 
tilization. The inflated spheres are 
about twice the size of the unburned 
particles. Furthermore, agglomeration is 
quite evident. 



24 





A Unburned 



300 



Scafe, /Am 



D Burned 



Scole, jim 



300 





5' Unburned 



JL 



100 



Scole, ^m 



E Burned 



100 



Scale, ^m 





C Unburned 



30 



Scole, ftm 



F Burned 

Cm=IOO g/rn^ 



if 



SccMa, fun 



FIGURE 19. - Pittsburgh coal dust (22-;xm), unburned and burned in 
50% 2. 



25 




^ Burned 







300 



J L 



Scale, ^m 




C Burned 
Cm*350 g/m^ 



300 



_i u 



Scale, ^m 



% 



*V' 












Burned o 

Cm=250 q/n? 



100 



Scale, ^m 



D Burned 

Cn,=350 q/tr? 





I L 



too 



Scale, fitti 



FIGURE 20. . Pittsburgh coal dust (22.^m), burned in 50% Oj. 



26 




600 



COAL DUST CONCENTRATION, g/m' 



FIGURE 21. - Flammability data from the 8-L chamber for 2-fim Pittsburgh coal dust in air. 



Particle Size Effects for Pittsburgh 
and Pocahontas Coals 

Figure 26 shows the effect of particle 
size on the flammability of Pittsburgh 
(35% volatility by the ASTM D3175 test) 
and Pocahontas (16% volatility) coals 
(15) . The Pittsburgh coal shows a con- 
stant lean flammable limit for particle 
sizes up to about 40 ym. The lean limit 
then rises, slowly at first and then more 
rapidly. Above 100 pm the dust becomes 
nonignitable even with a very strong ig- 
nition source (six matches). The Poca- 
hontas coal shows a slow rise in the 
lean limit with increasing particle size 
even for the finest sizes studied. At 
2 ym, it has a lean limit only slightly 



higher than that of Pittsburgh coal, but 
at 20 to 50 ym the limit is significantly 
higher. Above 80 ym, the Pocahontas coal 
becomes nonignitable. At the smaller 
sizes, the variation in the lean limit 
appears to be due to variations in the 
real volatility of the dusts at the high 
heating rates of flames compared to the 
standard ASTM volatility test. At the 
larger sizes, the lean limits for both 
coals increase because the particles are 
not able to devolatilize fast enough to 
feed the propagating flames (15). Con- 
sequently, a higher mass concentration of 
dust is necessary to provide the minimum 
amount of combustible volatiles at the 
lean limit. 



27 




A Unburned 



_l L 



300 



Scale, /ifn 




C Unburned 







30 



J L 



Scale, ftfR 





B Unburned 



100 



Scale, ^m 



D Unburned 



I L 



10 



Scale, ^m 



FIGURE 22. • Unburned Pittsburgh coal dust (2.,im). 



28 



g 




A Burned 

C„,=200 g/m^ 



300 




Scoie, /i.m 



C Burned 

Cm=200 g/m^ 



Scale, ^m 



100 





B Burned 
Cm=200 g/m^ 





L 



100 



Scoie, ^m 



D Burned 

Cm=200 g/m^ 





L 



30 



J L 



Scoie, /xm 



FIGURE 23. - Pittsburgh coal dust (2-/zm), burned in air. 



29 



kf,bUU 


1 


1 


1 


1 




1 1 1 1 

KEY 




2,400 


- 




D 


D 




D — D Gas 1 ^< 

.^^ Dust - ^^ Pyonneter 

X X Dust, 3X pyrometer 


- 


^ 2,200 
uJ 

01 

Z) 

fe 2,000 

UJ 
CL 

H 1,800 


- 




< 


N 
\ 
\ 

• 

• • 


\ ° 
\ 

\ n 

\ n D 

D X •^ ~-~. X 


- 


"^x 

X 


X 


1,600 

1 /ir\r\ 


- 






1 




2 

till 


- 



o 

i- 
< 
a: 

UJ 

a: 

Z) 

(/^ 

UJ 

q: 

CL 





FIGURE 
air. 



_ a w^ a . . _ 

• ^ ^ • * • • 

_ • / 

/• 

I _ ^ • I I I I I 



100 200 300 400 500 600 700 800 900 
COAL DUST CONCENTRATION, g/m^ 

24. • Flammability data from the 8-L chamber for 84-/xm Pittsburgh coal dust in 



Pocahontas Coal Dust in Air 

The explosion pressure variation with 
dust concentration for a size-classified 
Pocahontas seam coal is shown in fig- 
ure 27. This dust has a surface mean di- 
ameter Dg = 13 pm and a mass mean diame- 
ter D^ = 17 ym. Note that the lean limit 
is only slightly higher than for Pitts- 
burgh coal of similar size, but the maxi- 
mum pressures are significantly lower. 
Micrographs of this coal before explosion 
are shown in figures 28^-28(7. It looks 



very much like unburned Pittsburgh coal 
in that the surfaces exhibit sharp and 
angular features. The dust residues of 
Pocahontas coal after an explosion at a 
concentration of 450 g/m^ are shown in 
figures 28D-28F* The burned particles 
also look much like the Pittsburgh coal 
dust residues after explosions at simi- 
lar concentrations. There are extensive 
blowholes and cenospheres. Also evident 
in figure 28£' is some agglomeration of 
coal dust due to collisions while still 
in the plastic state. 



30 










/4 Unburned 



500 

J 1 



Scale, ftm 



D Burned 

Cm=300 g/rr^ 



500 



Scale:, ^m 





B Unburned 





1 u 



300 



Scale, /4.m 



£■ Burned 

Cm" 300 g/m^ 



300 



Scale, ^ 





C Unburned 



Scole. /iffi 



too 

J 



F Burned 

Cm=300 g/m^ 



K)0 



Scale, ^m 



FIGURE 25. - Pittsburgh coal dust (84-^m), unburned and burned in 
air. 



31 



600 



500 



o— ^ Pittsburgh coal, 6 matches 
X X Pocahontas coal, 6 matches 



400 



£ 

C7> 



1 300 



< 

LlI 

_l 



200 



00 



T 



I I ' ' 'I 



KEY 
-• Pittsburgh coal, 4 matches 



*2^ .-- — ' 




I > ■ ■ ■ I 



I I I I I I 



5 10 20 _ 50 

MEAN PARTICLE SIZE, D^, fjum 



100 200 



FIGURE 26. 
particle size. 



Lean flammable limit for Pittsburgh and Pocahontas coals as a function of 



Anthracite Dust in Air and in 50% O2 

Anthracite dust, which has a lower vol- 
atile content (7% by the ASTM D3175 test) 
than bituminous coal dusts, was also 
tested in the 8-L chamber. Anthracite 
dust is normally considered to be nonex- 
plosive in air. However, with a strong 
ignition source and heavy dust loading 
(400 to 700 g/ni3), this very fine dust 



(D3 = 5 ym and D^^, = 9 jjm) was able to 
produce explosions in air (21% O2). The 
pressures of these explosions are shown 
in figure 29, which shows both the lean 
and rich explosion limits of this dust 
in air. In a 50% O2 atmosphere, the dust 
becomes explosive at a much lower con- 
centration (150 g/m^), and it continues 
to be explosive with increasing dust 
concentrations, SEM micrographs of dust 



32 



< 

liJ 
IT 

(/) 
UJ 

q: 

Q. 



4 - yi • 

3 - 

- J 



100 200 300 400 500 

COAL DUST CONCENTRATION, g/m^ 



600 



700 



FIGURE 27. • Flammabiiity data from the 8-L chamber for 13-/im Pocahontas coal dust in air. 



residues from explosions in air and in 
50% O2 are shown in figures 30 and 31. 
The unburned anthracite dust shows a very 
broad size distribution (figs. 304-30C). 
Sharp and angular features are evident. 
The majority of burned particles do not 
look much different from those before the 
explosion. However, some of them showed 
definite changes, such as the appearance 
of small blowholes. Because this is not 
a bituminous coal and does not go through 
a plastic phase during devolatilization, 
the particles do not become rounded and 
do not form cenospheres. The fraction of 
the coal particles covered by microblow- 
holes is larger for coal particles in the 
50% O2 explosion than for that in air. 



appearance of the particles in fig- 
31c and 3lF seems to indicate lay- 



The 

ures 

ered structures. 



The smallest particles that are eas- 
ily visible in the unburned sample 
(figs. 30A-30C) are almost totally absent 
in the samples collected after explo- 
sions, especially for the 50% O2 atmos- 
phere. It is uncertain whether they were 
consumed by the explosion or were not 
sampled. However, it is unlikely that 
they totally escaped being sampled be- 
cause the same sampling method used in 
other experiments successfully collected 
small particles. 



33 





A Unburned 



300 



Scale, fim 



D Burned 
Cm=450 g/m3 



300 

J 



Scola, fi.m 





6 Unburned 



Scale, jxtn 



100 

J 



E Burned 
Cm=450 g/m^ 



too 



Scale, ^m 





C Unburned 



30 



Scale, ^m 



F Burned 



30 



Seal*, fiLtn 



FIGURE 28. - Pocahontas coal dust (13-fxm), unburned and burned in 
air.. 



34 



8 



o 


6 


K 




< 




cr 




LiJ 

a: 


5 


ZD 




CO 




CO 




UJ 




nr 




a. 


4 




100 200 300 400 500 600 700 800 900 

COAL DUST CONCENTRATION, g/m^ 

FIGURE 29. - Flammability data from the 8-L chamber for anthracite in air and in 50% O2. 



Graphite Dust in 50% O2 

To obtain combustible dusts of even 
lower volatile content, graphite powder 
and diamond dust were chosen. They have 
volatile contents of about 1/2% and 0%, 
respectively, and are allotropic forms 
of pure carbon. Graphite dust with D^ 
= 4 ym and D^^ = 6 pm exploded in a 50% 
O2 atmosphere. The explosion pressure 
variation with concentration is shown 
in figure 32. The micrographs of before 
and after explosion samples are shown in 



figure 33. There is not much difference 
in the appearance of the two samples. 
The graphite dust samples were amorphous 
graphite and, unlike the coal sam- 
ples studied previously, do not exhibit 
sharp angular features. However, again, 
the small particles so visible in the 
unburned sample (fig. 33^) are less evi- 
dent in the burned sample. It is quite 
possible that they are consumed in the 
explosion. No obvious blowholes or 
deformations of the remaining particles 
are evident. 



35 





A Unburned 



300 



Scale, ftm 



D Burned 



300 



Scale, /ifn 





B Unburned 



Scale, fifn 



100 
J 



£" Burned 



! ^ L 



100 



Scale, /Am 





C Unburned 



30 



JL I- 



Scale, ^m 



F Burned 



30 

-J 



Scale, fim 



FIGURE 30. - Anthracite dust, unburned and burned in air. 



36 



tmmm 





A Burned 
Cm=400g/nn5 



300 



Scate, fjum 



D Burned 
Cm =400 g/m^ 



300 

1 J . I 

Scole, ^ 





B Burned 

Cm=400 g/m^ 



Scole, jum 



E Burned 

Cn,=400 g/m' 



too 

J I 



Scole, ^m 





C Burned 
Cm=400 g/m^ 



O 30 

i I I 

Scale, /i.m 



F Burned 

Cm=400g/m* 



30 

I I I 

Scale, ^m 



FIGURE 31. - Anthracite dust, burned in 50% 0; 



37 




900 



DUST CONCENTRATION, g/m' 



FIGURE 32. - Flammability data from the 8-L chamber for graphite and diamond dust in 50% O2. 



Diamond Dust in 50% O2 

The explosion pressures for synthetic 
diamond dust (D^ = 2.3 ym and D^^ = 2.6 
ym) are shown in figure 32. A rather 
large explosion (pressure ratio = 8) 
was produced at a dust concentration of 
800 g/m^ in 50% O2 , using a strong igni- 
tion source of six matches. The dusts 
before and after explosion have the same 
angular appearance (fig. 34). There are 
some smooth, spherical particles in the 
after-explosion samples (especially in 
figure 3Ue) i but these are not diamond 
particles. Elemental distribution maps 



of the burned particles in figures 34z?- 
3Uf were made using lead and cerium X- 
rays , and these are shown in figures 34(7- 
34j. Comparing these maps with their 
respective micrographs shows that those 
particles having smooth surfaces were 
actually remnants from the electrical 
matches used to initiate the explosions. 
The X-ray spectrum of these match parti- 
cles as well as that of the diamond 
particles is shown in figure 35. The 
strong M-lines of lead near 2.4 KeV and 
the L-lines of cerium near 5 keV were 
used to generate the X-ray maps in fig- 
ures 346^-34j. The combustion of graphite 



38 





A l^burned 



300 

J 



Scole, itxn 



D Burned 

Cm=250 g/m3 



300 

1 I 1 

Scale, ^m 




B Unbumed 



Scale, /xm 



'"^ '^^^.W^*' 



£■ Burned 



100 

_. 1 



Scola, ^tn 





C Unbumed 



30 



Scale, /^m 



F Burned 
Cm=250 g/m^ 



30 

J 



Scole, ^tt\ 



FIGURE 33. - Graphite dust (4-^m), unburned and burned in 50% O2. 



39 






A Unburned 



I L 



100 



Scale, ^m 



D Burned 

_ I I 

Cm-8(X) g/m-' Scale, ftm 



100 



6 X-ray map 

of matches 



too 



Scale, /AID 












5 Unburned 



30 



Scole, /im 



F Burned 



30 



Cm=800 g/m' Scale, ;u.m 



H X-roy map 
of matches 



30 



Scole, /»m 






C Unburned 



Scole, /i.m 



F Burned 



Cm=800 g/m^ Scoio, ^m 



10 
J 



/ X-roy map 
of matches 



10 



Scats, |i^R) 



FIGURE 34. - Diamond dust (2-/im). A-C, unburned; D-K , burned in 50% O2; G-l, X-ray map of 
match particles using the Pb and Ce lines. 



40 



1 r- 



1 I ' — I — I — I — I — r 

/i Match 



T r- 



c 

a 

k_ 

.B 
o 

>■ 

(/) 

Z 
UJ 








■ " 



- 2- 




14 



6 8 10 12 

ENERGY, keV 

FIGURE 35. - X-ray spectra of diamond and match particles. 



16 



18 



20 



and diamond dusts must be quite similar 
to each other, but their residues are 
substantially different from those of 
volatile bituminous coal dusts. The an- 
thracite dust residue is intermediate in 
nature. 

Pittsburgh Coal and Rock Dust in Air 

To prevent and suppress coal dust ex- 
plosions, coal dust can be mixed with 
inerting agents which act as inhibi- 
tors. The one actually used in mines 
is rock. dust. The relative effective- 
ness of a range of chemicals that can be 
used for this purpose was studied by the 
Bureau of Mines, and details are found in 
the appropriate reports (13-14, 16-17). 
Pittsburgh coal dust and limestone rock 



dust (CaC03) mixtures containing 40% in- 
combustible (I) matter were tested in the 
8-L chamber, and the pressure versus con- 
centration data are shown in figure 36. 
Note that both the coal and total dust 
concentrations are shown on the horizon- 
tal axes. The incombustible content 
includes both the ash in the coal and the 
rock dust. Both the coal and rock dusts 
were minus 200 mesh (D < 74 pm) , and the 
mixture had D^ = 23 pm and D^^ = 40 ym. 
Also shown in the figure are the particle 
temperatures for these explosions. Com- 
paring figure 36 with figure 10, it can 
be seen that the explosion pressures and 
particle temperatures are lower for the 
40% incombustible mixture than for the 
pure coal dust. 



41 



UJ 

oc 

liJ 
Q. 

LiJ 

h- 

UJ 

_J 

o 

h- 

< 

Q. 



r 2.000 



COAL DUST CONCENTRATION, g/m^ 
100 200 300 400 500 600 700 



1,800 - 



1,600 



1,400 



g 
on 

UJ 

CO 
CO 
UJ 

a: 

Q. 








X* 




KEY 

X 3x pyrometer 
6X pyrometer 




200 400 600 800 1,000 

TOTAL DUST CONCENTRATION, g/m^ 



,200 



FIGURE 36. - Flammability data from the 8-L chamber for a Pittsburgh coal and rock dust 
mixture (40% I). 



Figures 37^-37c are micrographs of 
bef ore-explosion samples of this coal- 
rock dust mixture. Also shown are cal- 
cium K^ X-ray maps to the right of their 
respective micrographs. These maps can 
be used to identify the limestone rock 
dust (CaC03) particles in the dust mix- 
ture since only the rock dust parti- 
cles appear on the calcium X-ray maps. 
This strong calcium X-ray line at 3.7 keV 
is obvious in figure 38, which compares 
the spectrum of limestone rock dust with 
that of coal dust. Figure 39 shows the 
SEM micrographs of the rock-dust-treated 



coal dust after an explosion at a coal 
concentration of 400 g/m^. Also shown 
are the calcium K X-ray maps to the 
right of their respective macrographs. 
Notice that there are cenospheres and 
particles with blowholes, but a few of 
the particles are unaffected by the 
explosion and retain their unburned 
angular, sharp characteristic surfaces. 
By contrast, micrographs from explo- 
sions of the same concentrations of 
Pittsburgh coal dust (fig. 14) show that 
all the visible particles are extensive- 
ly deformed due to heating, reacting, 



42 



■i\*'<v, i, 



■WW- V ^^few' t 



■^:i^"r<' 



-■.«,#■ .-i"- 




A Unburned 



300 



Scale, ^m 



D X-rOf mop of rock 9 



300 



Scote:, (tm 





B Unburned 



100 



Scols, ^m 



£ X-ray map of rock o 



100 

J 1 



Scaie, fj.m 





C Unburned 



_1 L 



30 

_J 



Scole, ^m 



F X-ray map of rock 9 



30 

_J 



Scale, ^m 



FIGURE 37. - Unburned Pittsburgh cool and rock dust mixture 
(40% I). X-ray maps mode using the Co line. 



43 




4 5 6 7 8 

ENERGY, keV 

FIGURE 38. - X-ray spectra of limestone rock and coal particles. 



10 



outgassing, and devolatilization. The 
rock dust is believed to function mainly 
as a thermal inhibitor by absorbing heat 
from the explosion. The heat can be 
absorbed by the inert particle itself 
and/or by its endothermic decomposition 
to CaO and CO2. The decomposition prod- 
uct, CO2 gas, may also help to suppress 
the explosion, 

EXPERIMENTAL MINE DUST EXPLOSIONS 

Figure 40 shows an unburned mixture of 
coal dust with rock dust (60% I) , and 
figure 41 shows the residue from a full- 
scale mine explosion of the same dust 
in the Bruceton Experimental Mine (30). 



Also shown are corresponding calcium 
X-ray maps to identify the rock dust 
particles in the micrographs. The ap- 
pearance of the burned dust particles is 
very similar to that of those from the 
laboratory-scale experiment (fig. 39) , 
showing good correlation between the ex- 
tremes in scale of the experiments. This 
amount of rock dust (60% I) is less than 
the amount necessary to totally inert the 
coal-rock mixture, and therefore the 
flame propagated the entire length of the 
dusted zone in the mine. The dust sam- 
ples for SEM analysis were collected by 
an automatic sampling system during the 
passage of the propagating flame. 



44 





Coal Cm:400 q/vc? 



300 

J 



Scale, fjjm 




B Burned 

Coat Cm=^400 q/tt? 



iOO 



Scaie, /im 



D X-ray mop of rock 



300 

J 



ScQie, ft.m 




E X-ray mop of rock 9 



100 



Scole, ^m 





C Burned 



30 



Coal C„ =400 g/m^ Scol«, /uim 



F X-ray map of rock 



30 



Scale, ^m 



FIGURE 39. - Pittsburgh coal and rock dust mixture (40% I), burned 
in air. X-ray maps made using the Ca line. 



45 





A Unburned 



Scale, ^m 



too 

_J 



X-roy map of rock 



100 



Scole, ^m 





B Unburned 



30 



Scale, >tm 



X-roy map of rock 9 



30 



Scole, ^m 





C Unburned 







30 



Scale, /ifK 



f X-ray mop of rock 



30 



Scole, fifn 



FIGURE 40. - Unburned Pittsburgh cool and rock dust mixture 
(60% I). X-roy mops mode using the Co line. 



46 





4 Burned 



Scole, fj.m 



too 

-J 



Z? X-ray mop of rock 9 



Scole, fj-m 



too 

J 





B Burned 



30 
-J 



ScQie, ^m 



£ X-roy map of rock o 



30 

_J 



Scole, /i.m 




C Burned 



30 

J 




F X-ray mop of rock 



Scal«. fim 



30 



Scale, fi.m 



FIGURE 41. - Pittsburgh coal and rock dust (60% I), postexplosion 
sample from the Bruceton Experimental Mine. X-ray maps made using 
the Co line. 



47 



MINE EXPLOSION DISASTER INVESTIGATIONS 



DUST EXPLOSIONS IN A 1.2-L FURNACE 



The Bureau's scanning electron micro- 
scope has also been used to assist in 
the investigation of mine explosion dis- 
asters at the request of the Mining Safe- 
ty and Health Administration (MSHA) of 
the Department of Labor, In one disaster 
(10) in which 15 miners were killed, a 
mine explosion occurred following a meth- 
ane outburst in a Colorado mine, MSHA 
investigators suspected that the igni- 
tion source for the explosion might have 
been an electric arc in an improperly- 
assembled explosion-proof compartmenf on 
a continuous mining machine. Dust sam- 
ples collected from within this com- 
partment were examined with the SEM. 
The presence of burned dust particles 
in the samples, together with other 
evidence, led the MSHA investigators 
to conclude that the probable ignition 
source was an arc in the improperly 
assembled compartment leading to a propa- 
gating methane explosion. 



Figure 42 is a schematic of a new 1.2-L 
furnace developed by Conti and others (8) 
to measure the thermal ignitability of 
dust clouds. The ceramic combustion 
chamber was wrapped with Nichrome heating 
wire and then surrounded with insulation. 
Four access holes pass through the fur- 
nace tube. Two are used for thermocou- 
ples, a 12.5-mil (320-]jm) Chrome 1-Alumel 
type K thermocouple at the wall of the 
furnace for control of the furnace tem- 
perature, and a 1-mil (25-ym) platinum- 
rhodium type S thermocouple positioned in 
the center of the furnace to observe 
rapid changes in temperature at ignition. 
The other two holes are used for the 
microprocessor-controlled rapid-sampling 
system, which will be described in de- 
tail in a later paragraph. Probing ver- 
tically along the furnace tube with a 
thermocouple indicated a relatively uni- 
form axial temperature profile along the 
tube length, A reasonably uniform dust 



Burst diaphragm 





Scale, cm 



/-Air dispersion 
^— tank 



Solenoid 
valve 



FIGURE 42. - 1.2-L furnace and rapid sampling system (8). 



48 



distribution within the furnace tube was 
also measured by a miniature dust probe 

a). 

The dust to be tested is placed in a 
brass dispersion receptacle at the base 
of the furnace. To minimize preheating 
of the dust, the receptacle was inserted 
into the furnace just before dispersion. 
The receptacle is also fitted with a 
ceramic cloth sleeve which serves as in- 
sulation and as a seal along the recepta- 
cle's length. The nozzle of the dust 
receptacle contains a number of small 
holes through which the dust is ejected 
by a 30-msec pulse of air from a disper- 
sion tank. In these furnace tests, the 



maximum possible exposure time for the 
dispersed dust was several seconds, be- 
fore the dust would settle. The cri- 
terion for ignition in the tests was the 
rupture of the diaphragm (at 0.2 to 0.3 
bar) and flame emitting from the top of 
the furnace. 

The data in figures 43 and 44 are used 
with the kind permission of R. S. Conti 
(8^). Figure 43 shows pressure (absolute) 
and temperature (from the 1-mil thermo- 
couple) traces for a thermal ignition of 
Pittsburgh coal at a concentration of 260 
g/m3 in the 1.2-L furnace. The furnace 
temperature was 600° C, slightly above 
the minimum autoignition temperature for 




fi 300 



0.4 0.6 
TIME, sec 



FIGURE 43. - Pressure and thermocouple recorder traces from an explosion in the 1.2-L furnace. 



49 



1,100 




500 



KEY 
• Ignition 
o Nonignition 



Thermally ignitable 



• — 



A t_ 



jj 



Not thermally ignitable 



KX) 



200 



300 



400 



DUST CONCENTRATION, g/m 



FIGURE 44. - Dust cloud thermal ignition data from 
the 1,2-L furnace for 55-fxm Pittsburgh coal in air, 

this dust. There is a drop in tempera- 
ture as the dust is dispersed into the 
furnace, but the dust-air mixture temper- 
ature rises quickly to the set tempera- 
ture of the furnace walls and then ther- 
mally ignites at about 0.8 sec after 
dispersion. Ignition is shown by the 
rapid rise in pressure and temperature, 
followed by a sudden drop in both as the 
diaphragm ruptures and the combustion 
products vent and cool. 

Figure 44 shows the thermal autoigni- 
tion temperature (AIT) data for the size- 
classified Pittsburgh seam coal dust (35% 
volatility, D3 = 55 ym, and D^^ = 62 ym) 
dispersed in the preheated 1.2-L furnace. 
The concentrations are the nominal con- 
centrations, namely the mass of dust 
divided by the chamber volume. No cor- 
rections are included for the reduction 
in the mass concentration of oxygen at 
the elevated temperatures. The minimum 



AIT is 550° C, and that minimum value is 
approached asymptotically at the higher 
dust concentrations. The general shape 
of the curve for coal is similar to that 
observed for other hydrocarbons, solid or 
liquid. 

Dust residues as well as gaseous prod- 
ucts were sampled with the apparatus 
shown in figure 42. Sampling is accom- 
plished by plunging an evacuated glass 
tube (sealed with a rubber stopper) 
against the sharp needle. When the stop- 
per is pierced by the needle probe, gase- 
ous and dust samples are drawn into the 
sampling tube. The mechanism that actu- 
ates the sampling tube is controlled by a 
microprocessor. The timing and the dura- 
tion of sampling are precisely controlla- 
ble. For 55-ym Pittsburgh coal dust at a 
concentration of 260 g/m^ and at an ini- 
tial furnace temperature of 600° C, sam- 
ples were collected while the dust was 
being heated and after it ignited. SEM 
micrographs of the unburned dust are 
shown in figure 45. The heated dust par- 
ticles (fig. 46) were sampled at a time 
of 0.76 sec after dispersion (see fig. 
43), and the burned particles (fig, 47) 
were sampled at a time of 1,06 sec, which 
was just after ignition and diaphragm 
rupture. In the preheated system (fig, 
46) , there is clearly substantial devola- 
tilization of many particles prior to 
ignition. Although some particles are 
unchanged, others contain plentiful blow- 
holes, and many particles appear as 
strongly devolatilized as the postigni- 
tion residues. These data suggest that 
the autoignition process involved devola- 
tilization prior to the thermal ignition 
of the gas phase mixture of coal tar 
volatiles in air. The postexplosion sam- 
ple (fig, 47) contains mostly burned par- 
ticles, but a few particles appear almost 
unchanged. This is due to the fact that 
the diaphragm ruptured before the explo- 
sion propagated throughout the entire 
chamber, thus leaving a few relatively 
unchanged particles. 



50 




A Unburned 
Pittsburgh coal 



B Unburned 
Pittsburgh coal 



Scots, ^m 




30 

1 



ScQift, u.fn 




t 



30 



C Unburned 

Pittsburgh coal scale, ^m. 

FIGURE 45. - Unburned Pitts- 
burgh cool dust (55-fim). 



LASER PYROLYSIS OF COAL DUST 

The dust residues from the 8-L flamma- 
bility apparatus described in the previ- 
ous sections gave information about the 
combustible dusts after the explosion 
process was completed. The explosion 

event occurs in an experimental time x 

'^ ex p 

of seconds. During that time interval, 
the coal particles are subjected to in- 
tense energy fluxes, varying from about 
30 to over 200 W/cm2. The results ob- 
servable after an explosion are integral 
effects of combustion over the time Xgj^p. 

One would like to be able to observe 
the coal particle at intermediate stages 
during the evolution of the explosion. 
This is very difficult in the 8-L appara- 
tus because one cannot stop the progres- 
sion of an explosion once it is initi- 
ated. A separate project was therefore 
initiated to address the question of coal 
pyrolysis under rapid and intense heat- 
ing, A high-energy infrared CO2 laser 
capable of delivering up to 200 W of con- 
tinuous radiation at a wavelength of 10.6 
ym or operating in the pulse mode for 
durations from 1 msec to tens of seconds 
is used to simulate the rapid heating 
condition in an explosion. A single 
pulse of laser radiation is directed at a 
small quantity of coal dust supported on 
a substrate, and the peak energy and dur- 
ation of the laser pulse are varied to 
simulate the rapid heating of a coal par- 
ticle in an explosion. The brief appli- 
cation of the pulse allows one to isolate 
the particle at different stages of the 
combustion process. Preliminary results 
in which an unfocused laser beam at ap- 
proximately 80 W/cm2 is operated in 
pulses of 70 to 1,000 msec are described 
here. The heated coal particles were ex- 
amined in the SEM, and the results are 
shown in figure 48, where they are com- 
pared with the original, unheated dust. 
This was the size-classified Pittsburgh 
coal with D5 = 84 pm and D^^ = 89 pm, 
shown in figure 9. 



51 





/i Heated, T^eOCC L-___j___J° 
Cm=260 g/m^ Scale, ^m 



/? Heated, 1=600" C 
Cm=260 q/n? 



Scoie, /ifn 



100 

J 




B Headed, T^SOCC 
Cm«260 g/m^ 



30 



Scale, ^m 




£ Heated, 1=600° C 
Cm =260 g/m^ 



30 

_J 



Scale, /ttm 





C Heated, 1=600° C 
C^=260 g/m^ 



30 



Scole, /urn 



/=" Heated, T^eOO'C 
C,„=260 g/m' 



30 



Scole, ftm 



FIGURE 46. - Pittsburgh cool dust (55-;tm), heated in the 1.2-L fur- 
nace but before thermal ignition. 



52 





A Burned, T=600°C l___l__-J° 
Cm*260g/m^ Scale, ^m 



O Burned, T=600°C ^ ^ ^f 
Cn,=260 g/m^ Scole, ^m 





5 Burned, T^GOO'C L___i___-3° 
Cm=260 g/m^ Scoks, ^m 



£ Burned, T=600°C l___j____j° 
Cm* 260 g/m^ Scale. /^m 





C" Burned, 1=600° C 
Cm=260 g/m^ 



O 30 

I 1 I 1 

Scale, fun 



F Burned, T^eOCC O 

L 



30 

_i I 



Cm" 260 g/m Scrte, ^ 



FIGURE 47. - Pittsburgh coal dust (55-fim) after an explosion in the 
1.2-L furnace. 



53 





A Un heated 







X 



too 



Scale, /xm 




C Heated, 
t=500 msec 




B Heated, 
t=70 msec 



1 



100 



D Heated, 
tH, 000 msec 



100 



Scale, ftm t.^i,uuu msec Scale, y,m 

FIGURE 48. - Laser-pyrolyzed 84-/im Pittsburgh coal at various heating times. 



msec (fig. 48b), the particles 
the first stage of devolatili- 
The coal became plastic and 
and the characteristic sharp 
of the particle surfaces became 
With a longer pulse time (500 
the coal particles became more 
fluid, and surface tensional forces pro- 
duced the surface of minimum energy which 
is a sphere (fig. 48C). At even longer 
pulse lengths (1,000 msec, fig. 48Z?), the 



At 70 
showed 
zation. 
fluid, 
angles 
smooth, 
msec) , 



volatiles began to ignite owing to the 
intense heating effect of the laser 
beam. Blowholes and the other familiar 
features of coal dust residue from a vio- 
lent explosion in the 8-L apparatus were 
produced. Further development in this 
project and the SEM study of the coal 
dust residues should lead to better 
knowledge of the detailed combustion and 
explosion processes of coal. 



54 



SLOW HEATING OF COAL DUST 

The surface characteristics of coal 
particles also change when the dust is 
heated slowly. To examine this effect, 
20-g samples of Pittsburgh seam coal dust 
were placed in a preheated furnace at 
120°, 140°, 300°, and 500° C. When the 
dust reached the furnace temperature, a 
sample was taken for SEM analysis, and 
the results were compared to the origi- 
nal, unheated dust. The dust studied was 
200 by 100 mesh coal with an average size 
of about 100 ym. 

At 120° and 140° C, the SEM micrographs 
of the heated dust showed no obvious 
changes from the original dust. Fig- 
ure 49 shows the changes that occurred at 
the higher temperatures. On the left 
(figs, 49^-49(7) are the original, unheat- 
ed coal particles at three magnifica- 
tions. The center column (figs, 49Z?-49f) 
shows changes in the surfaces of the par- 
ticles at 300° C, The right column 
(figs. 496^-49j) shows the rounding of the 
particles as the coal becomes plastic at 
500° C. 

SURJACE CHARACTERISTICS OF BITUMINOUS 
AND LIGNITE COALS 

The surface features of unheated Pitts- 
burgh coal and a lignite coal are quite 
different, as can be observed in fig- 
ure 50. The bituminous coal has very 
sharp angular edges but smooth surfaces. 
The lignite coal particles do not show 
any well-defined edges, and the sur- 
faces are rough and show extensive pore 
structures. The difference in surface 
features could be the reason for the dif- 
ferent properties of the two coals. Lig- 
nite coals are more susceptible to spon- 
taneous combustion (20), which is due to 
the oxidation of coal by air. The re- 
lease of heat in the oxidation process 
raises the temperature of the coal and 
eventually results in sustained combust- 
ion. The larger surface area per unit 



mass of lignite coal and the possible 
chemical structure differences of the 
coals are responsible for the increased 
susceptibility to spontaneous combustion. 

CHEMICALLY TREATED WOOD 

Timbers used in mines are some- 
times chemically impregnated with fire- 
inhibiting chemicals for improved fire 
resistance (37), The impregnation is 
accomplished by immersing timbers in a 
solution under hydrostatic pressure. One 
such commercially available timber treat- 
ment (NCX) contains a phosphorus com- 
pound. When a sample was made from one 
of these chemically treated oak timbers 
and examined by the SEM using the energy- 
dispersive X-ray analyzer (EDS), the only 
prominent X-ray signal detected was the 
phosphorus X-ray line. The relative con- 
centration profile of the phosphorus- 
containing chemical was determined by 
plotting the relative intensity of the 
phosphorus X-ray signal as a function of 
distance from the surface of the sample. 
The result is shown in figure 51, which 
includes a photograph of the wood cross 
section and a corresponding graph of the 
relative distribution of phosphorus in 
the treated wood. The graph shows a 
gradual decrease in the amount of phos- 
phorus from the surface (left side of 
figure) to the interior (right side) of 
the wood sample. Another sample from a 
piece of this chemically treated wood 
that was charred ( 21 ) by exposure to an 
intense heat lamp for 1 min was similarly 
prepared and examined. The resulting 
relative concentration profile and corre- 
sponding photograph are shown in fig- 
ure 52. There seems to be an enrichment 
of the phosphorus chemical on the sur- 
face, which might be due to migration to 
the surface of the phosphorus compound 
during heating. This may be a contrib- 
uting factor to the well-known fire re- 
sistance characteristic of a phosphorus- 
treated material (25, ch. 2). 



55 






A Unheated, 



300 



Scale, ^m 



D Healed, 
1^300 "C 



300 

-1 I 



Scoie^^m 



G Heated, 
T=500°C 



300 



ScQte, /im 






B Unheated, 
T=25''C 



iOO 



Scote, ^^x^ 



E Heated, 
T=300''C 



too 



Scale, ftm 



H Heated, 
T-SOCC 



Scale, ^m 



iOO 
_J 






C Unheoted, 

T=25»C 



30 



Scale, ^m 



F Heated, 
T=300°C 



30 



Scole, ^m 



/ Heated, 
T^SOO'C 



30 



Scote, ^m 



FIGURE 49. - Pittsburgh coal dust slowly heated in a furnace at various temperatures. 



56 





A Unbumed Pittsburgh lOO £» Unburned lignite <? 



bituminous coa! 



J J 

Sca!«, yu.m 



coal 



too 



Scale, )im 





B Unburned Pittsburgh 9 
bituminous coal 



300 



Scaie, faf\ 



E Unburned iignite 
coal 



300 



Scold, fj.tt\ 





C Unburned Pittsburgh ° ^ _ ^° P 
bituminous coal scai«.fiin 



F Unburned lignite ? 



coal 



300 

J 



Scot*, ^f» 



FIGURE 50. - Comparison of surface characteristics of bituminous 
and lignite coals. 



57 




DISTANCE, mm 

FIGURE 51. - Relative distribution of phosphorus as a function of distance from the surface, in 
oak timber chemically treated with NCX inhibitor. 

DISCUSSION 



Microscopic observations such as those 
in the present report should certainly 
contribute to improved theories of com- 
bustion. Unfortunately, only a few re- 
searchers have studied the true micro- 
scopic details of combustion events in 
dust explosions. Smoot, Horton, and 
their colleagues ( 18 , 33-34) published 

micrographs of Pitts- 
particles before and 
the flame front of a 
coal dust-air burner flame. During the 
short time (about 50 msec) that the par- 
ticles traversed the flame front, they 
lost their original sharp edges and angu- 
lar appearance and became more spherical. 
Many particles showed blowholes due to 



scanning electron 
burgh seam coal 
after traversing 



escaping volatiles. From the SEM micro- 
graphs, the particles did not appear to 
change much in size during the traverse 
of the flame front, but a Coulter Counter 
size analysis of collected particles 
showed a slight increase in average size. 

Similar results were found by Milne 
and Beachey (27), who also used a flat 
flame burner to study coal dust flame 
propagation. Their SEM micrographs also 
show rounded particles with extensive 
blowholes. 

Lightman and Street ( 24 ) extensively 
studied coal particles heated in fur- 
naces, shock tubes, and large combustors. 



58 








1.0 



c/) 

IJJ 

I- 



liJ 
> 



UJ 







\ 


-Char 

\ 


Ted— 

\ 


\ 






1 












— 


— 










• 


• 


• 


• 


m — 


• 


• 


— 



5 — 



8 



10 



DISTANCE, mm 



FIGURE 52. - Relative distribution of phosphorus as a function of distance from the surface, in 
chemically treated oak timber after charring. 



They observed the collected particles 
with both electron and optical micros- 
copy. In the drop tube furnace, 60- to 
110-ym particles were heated in air 
as they fell through the furnace in a 
period of about 1 sec. At furnace tem- 
peratures of 400° to 500° C, they ob- 
served internal gas bubbles in the parti- 
cles due to the evolving volatiles and 
some blowholes as the volatiles escaped 
the surfaces of the particles. At higher 
temperatures (up to 1,200° C), various 
types of cenospheres (29) were observed. 
These particles were sometimes twice 



their original size. The lower tempera- 
ture furnace results are comparable to 
the SEM micrographs from the 1.2-L fur- 
nace (figs. 46-47). The high-temperature 
results are similar to the postexplosion 
residue for 84-]jm Pittsburgh coal from 
the 8-L chamber (fig. 25). 

Lightman and Street ( 24 ) used shock 
tubes to study the rapid heating of 
coal particles in atmospheres of air and 
nitrogen. Cenospheres were observed for 
temperatures of 900° to 1,300° C behind 
the reflected shock wave. The residues 



59 



from their large combustor showed four 
types of char: solid particles, lacy 
cenospheres , balloon cenospheres , and 
thick -walled cenospheres. The internal 
structure of the cenospheres was studied 
by embedding the particles in a resin, 
and then sectioning the sample and ob- 
serving it with an optical microscope. 

In a later study. Street, Weight, and 
Lightman (36) studied the structural 
changes in pulverized coal during rapid 
heating in the drop-tube furnace in at- 
mospheres of air and nitrogen. The 
structures of the chars formed in the ox- 
idative and inert gases were different. 
The particles heated in nitrogen tended 
to be more open and form more "balloons" 
or cenospheres; they also had smaller 
pores than the particles heated in air. 
Swelling of the coal particles was ob- 
served to be influenced by the duration 
and temperature of the experiment. They 
also found that, with the exception of 
fusain (which formed solid particle 
chars), it was impossible to relate ex- 
actly the char types to particular litho- 
types of the original particles. 

Seeker and others (31) also studied the 
behavior of individual particles (40 and 
80 pm) passing through a heated gas zone, 
created by a lean methane-air flame. 
They collected particles for SEM analysis 
and also used holography to study the 
devolatilizing particles in situ. Par- 
ticle heat-up times were of the order of 
105 ° C/sec. For bituminous coals, they 
observed volatiles jetting from the par- 
ticles and an increase in particle size. 
Blowholes were also observed in the col- 
lected particles. For anthracite, they 
did not observe the volatile jets. 

McLean, Hardesty, and Pohl (26) also 
observed the pyrolysis of coal particles 
(about 100-ym size) by injecting them 
into the centerline of a high-temperature 
reactor fueled by a flat-flame gas burn- 
er. They used high-magnification shadow- 
graphs of the in situ particles and 
micrographs of the collected particles. 
They observed the generation of volatiles 
from the bituminous coal particles with 
the shadowgraph. The collected particles 



appeared to have melted and swelled, sim- 
ilar to those in figure 25. 

Helwig ( 12 ) studied the rapid (»1,000° 
C/sec) pyrolysis of narrow size distribu- 
tions of coal dusts in an inerted fur- 
nace. For 20- to 30-ym particles at a 
furnace temperature of 1,000° C, a "live- 
ly bursting" and devolatilization is ob- 
served. For larger (60- to 75-)jm) parti- 
cles, a swelling of the particles is ob- 
served as they become plastic, but the 
heating rate is not sufficient to rupture 
the particles. Helwig suggests that un- 
der a given condition, only coal parti- 
cles smaller than a certain size could 
attain a high enough heating rate to be- 
come liquid and allow a high enough gas 
pressure to build up to rupture the liq- 
uid surface as resolidification was 
occurring. 

The Bureau's systematic microscopic in- 
vestigations of postexplosion dust resi- 
dues have led to similar observations. 
The burned bituminous coal dusts are usu- 
ally significantly larger than the un- 
burned particles. The surfaces of these 
burned char residues are rounded and 
smoothed, in contrast to the angular fea- 
tures and sharp edges of the unburned 
coal. The burned particles are pock- 
marked with blowholes, and some are blown 
into large cenospheres. Clearly, the 
coal particles that generated the dust 
explosions were subjected to rapid heat- 
ing, causing them to pyrolyze and to 
soften or melt into a plastic phase. The 
heavier, higher-molecular-weight pyroly- 
sis products are liquid, and surface ten- 
sion forces cause them to assume a spher- 
ical shape. However, at the same time, 
the lighter-molecular-weight products of 
pyrolysis are devolatilizing (38). The 
internal pressures resulting from the 
devolatilization of these lighter compo- 
nents generate bubbles in the liquid mass 
of the heavier components. But the 
liquid mass is simultaneously reacting 
internally, and at about the same time 
that gas bubbles are appearing and burst- 
ing, competing condensation reactions 
occur in the liquid that generate an in- 
ert solidifying char matrix from the 
heavier components. The net results are 



60 



the deformed, swollen, char residues 
pockmarked with "frozen" blowholes of 
various shapes and sizes. 

If bituminous coal particles come into 
contact with one another during their 
short-lived plastic stage, they will 
stick together to form large agglomer- 
ates. At high dust concentrations, the 
collision frequency between particles in- 
creases and the probability of collision 
during the short time the particles are 
in the plastic stage becomes high. The 
residues observed after explosions at 
high concentrations are large, coked, 
agglomerated masses that have little 
resemblance in size or shape to the orig- 
inal coal particles. 

The temperature-time histories experi- 
enced by the coal particles during such 
explosions involve temperatures in the 
range of 1,500 to 2,000 K for times of 
the order of several tenths of a second 
(_5 , 13 ) , although the time the particle 
spends in the flame front is much short- 
er. The corresponding energy flux to the 
particles from flame front heating is in 
the range of 30 to 300 W/cm2. The ef- 
fects displayed in the SEM micrographs of 
residues from the 8-L chamber explosions 
are therefore integral effects that occur 
over relatively long time scales compared 
to the reaction time scales within the 
active flame front. It is possible to 
simulate the coal pyrolysis and devola- 
tilization processes occurring within a 
flame front on a time scale that matches 
the coal particle residence time and 
heating flux and that avoids the subse- 
quent exposure in the burned gas fire- 
ball during the longer time scale. This 
has been done with the high-power CO2 
laser operating at a flame flux level of 
approximately 80 W/cm2 in a pulse mode 
that controls the exposure time, 

A theory ( 15 , 17) to describe the de- 
volatilization and combustion of coal 



dusts based on results obtained from the 
electron microscopic investigation of 
dust residues as well as on other flam- 
mability data was developed, and good 
agreement with experimental data was 
achieved. 

The contribution of microscopy, and 
electron microscopy in particular, to the 
science of combustion can be extremely 
valuable in avoiding errors in theory. 
Consider, for example, recent studies 
( 22 , 32 ) on the ignition behavior of coal 
particles (3 to 25 ym in size) in a shock 
tube at temperatures of 1,700 to 2,200 K. 
In those studies, the change in light 
transmission through the dust cloud on 
the time scale of milliseconds was used 
to follow the reaction rate, A surface 
oxidation rate was inferred, using a 
shrinking sphere model for the particles 
in the burning dust cloud. As a result, 
activation energies and reaction orders 
were inferred, with the assumption that 
the optical diameter of each particle 
diminished uniformly as it reacted. How- 
ever, direct microscopic evidence of many 
other investigators, noted previously in 
the Discussion section, showed quite 
clearly that in the first several milli- 
seconds the coal combustion process pro- 
ceeds with essentially no change in the 
particle diameter, or with an increase in 
diameter. This initial process involves 
the devolatilization of the particle, 
which leaves a charred residue that is at 
least the same size as the original coal 
particle from which it was generated. In 
particular, Lightman and Street ( 24 ) ob- 
served the formation of cenospheres from 
bituminous coal particles in a shock tube 
at similar temperatures. Obviously, the 
available microscopic evidence contra- 
dicts the shrinking sphere model. The 
observed light transmission changes were 
thus probably unrelated to the rate of 
combustion reaction and are most likely 
caused by other processes. 



61 



CONCLUSION 



The SEM together with its X-ray acces- 
sory equipment is very useful in coal 
dust explosion research. It provides in- 
formation concerning the heating, devola- 
tilization, and combustion of coals and 
other carbonaceous dusts. Dusts of dif- 
ferent volatile content behave different- 
ly, as evidenced by the resulting surface 
characteristics. In addition to combus- 
tible dusts, the SEM is useful in the 



study of inhibitors and for qualitative 
and semiquantitative chemical analyses. 

The SEM is also a useful tool in inves- 
tigations of mine explosion disasters. 
Comparison of particles collected at 
disaster sites with those from laboratory 
explosion studies assists in determining 
the extent and causes of disasters. 



REFERENCES 



1, American Society for Testing and 
Materials, Standard Test Method for Vol- 
atile Matter in the Analysis Sample 
of Coal and Coke. D3175-77 in 1981 An- 
nual Book of ASTM Standards, Part 26: 
Gaseous Fuels; Coal and Coke; Atmospher- 
ic Analysis, Philadelphia, Pa,, 1981, 
pp, 372-375, 

2, Reiser, A, Concepts of Modern 
Physics, McGraw-Hill, New York, 1963, 
405 pp, (ch, 3), 

3, Born, M, , and E, Wolf. Principles 
of Optics, Pergamon Press, New York, 5th 
ed,, 1975, pp. 333-335, 396, 414-419. 

4, Cashdollar, K. L. Three Wavelength 
Pyrometer for Measuring Flame Tem- 
peratures. Appl, Opt,, v, 18, 1979, 
pp, 2595-2597. 

5, Cashdollar, K, L, , and M, Hertz- 
berg, Infrared Pyrometers for Measuring 
Dust Explosion Temperatures, Opt, Eng, , 
V, 21, 1982, pp. 82-86, 

6, Cashdollar, K, L, , M, Hertzberg, 
and C, D, Litton (assigned to U,S, De- 
partment of the Interior), Multichannel 
Infrared Pyrometer, U,S, Pat, 4,142,417, 
Mar, 6, 1979, 

7, Cashdollar, K, L, , I, Liebman, and 
R, S, Conti, Three Bureau of Mines Opti- 
cal Dust Probes. BuMines RI 8542, 1981, 
26 pp. 

8, Conti, R. S., K. L. Cashdollar, 
I, Liebman, and M, Hertzberg, Thermal 
Ignition of Dust Clouds, Pres, at Fall 



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